Nitride semiconductor device

ABSTRACT

A nitride semiconductor device including a light emitting device comprises a n-type region of one or more nitride semiconductor layers having n-type conductivity, a p-type region of one or more nitride semiconductor layers having p-type conductivity and an active layer between the n-type region and the p-type region. In such devices, there is provided with a super lattice layer comprising first layers and second layers which are nitride semiconductors having a different composition respectively. The super lattice structure makes working current and voltage of the device lowered, resulting in realization of more efficient devices.

TECHNICAL FIELD OF THE INVENTION

This invention relates to a device provided with a nitride semiconductorIn_(x)Al_(y)Ga_(1-x-y)N (0=x, 0=y, x+y=1) including light emittingdevices such as LED (light emitting diode), LD (laser diode) and SLD(super luminescent diode), solar cells, light receiving devices such asoptical sensors and electronic devices such as transistors and powerdevices.

BACKGROUND OF THE INVENTION

Nitride semiconductors have been recently produced as materials used fora high bright blue LED and a pure green LED, a full color LED displayand a traffic signal LED. Such LEDs are provided with an active layer ofSQW (Single Quantum Well) or MQW (Multi Quantum Well) where the welllayer is made of InGaN and positioned between a p-type nitride layer andan n-type nitride layer to form a DH (Double Hetero) structure. Thewavelength of the blue or green light emitting from the active layerdepends on a ratio of In in the InGaN well layer.

The inventors have first realized laser emitting by using the abovenitride materials and reported it in Jpn. J. Appl. Phys. 35(1996)L74 andJpn. J. Appl. Phys. 35(1996)L217. The laser device comprises the DHstructure where the active layer is MQW having InGaN well layers andshowed the following data:

Threshold current: 610 mA;

Threshold current density: 8.7 kA/m2;

Wavelength: 410 nm

(pulse width 2 μm and pulse cycle 2 ms)

The inventors have further improved the laser device and reported it inAppl. Phys. Lett. 69 (1996)1477. The laser device comprises a ridgestrip structure formed on apart of p-type nitride semiconductor andshowed the following data.

Threshold current: 187 mA;

Threshold current density: 3 kA/m2;

Wavelength: 410 nm

(pulse width 2 μm, pulse cycle 2 ms and duty ratio: 0.1%)

The inventors have first succeeded in CW (Continuous-Wave) Oscillationor Operation at room temperature and reported it in Gijutsu-Sokuho ofNikkei Electronics issued on Dec. 2, 1996, Appl. Phys. Lett. 69 (1996)and Appl. Phys. Lett. 69 (1996)4056.

The laser diode showed a lifetime of 27 hours at 20° C. under thethreshold current density of 3.6 ka/cm², the threshold voltage of 5.5 Vand the output of 1.5 mW.

On the other hand, the blue and green LED of nitrides showed a forwardcurrent (If) of 20 mA and a forward voltage (Vf) of 3.4 to 3.6 V whichare higher by 2 V or more than those of red LEDs made of GaAlAssemiconductors. Therefore, further decrease of Vf in the blue and greenLED was required. Additionally, there was required an effective LD whichcan decrease the threshold current and voltage to get a longer lifetimeof CW operation at room temperature, because the conventional LD stillhad a higher threshold current and voltage.

The inventors have gotten the idea that technology of decreasing thethreshold in LDs was applicable to LEDs in order to decrease the Vf.Therefore, a first object of the present invention is to decrease thethreshold current and voltage of nitride semiconductor LDs and realize alonger lifetime of CW operation.

In the specification, it should be understood that the general formulae:In_(x)Ga_(1-x)N and Al_(y)Ga_(1-y)N show chemical atoms which compose ofnitride layers and therefore, even if different layers are representedby the same formula, the different layers do not necessarily have thesame composition, that is, the same x or y does not mean the same ratio.

DISCLOSURE OF THE INVENTION

According to a first aspect of the present invention, there is provideda nitride semiconductor device comprising a p-type region comprising oneor more p-conductivity semiconductor layers of nitride, a n-type regioncomprising one or more n-conductivity semiconductor layers of nitrideand an active layer of a nitride semiconductor which is positionedbetween said p type region and said n type region, at least one layer ofsaid p type region being a super lattice layer comprising first thinlayers of nitride and second thin layers of nitride, said first layershaving different composition from those of said second layers and thefirst and second thin layers being laminated alternately.

The super lattice structure can make the nitride layers improved incrystallinity and then make the nitride layers decreased in resistivity,resulting in smaller resistance of the p-type region and higher powerefficiency of the device.

In the present invention, the p-type region means a region comprisingone or more nitride semiconductor layers between an active layer and ap-electrode while the n-type region means a region comprising one ormore nitride semiconductor layers between the active layer and ann-electrode.

According to a second aspect of the present invention, there is provideda nitride semiconductor device having an active layer made of a nitridesemiconductor between the n-type region of one or more nitridesemiconductor layers and the p-type region of one or more nitridesemiconductor layers, at least one semiconductor layer in the p-typeregion or the n-type region is a super lattice layer made by laminatingfirst layers and second layers which are made of nitride semiconductor,respectively, and have different constitutions from each other.

The super lattice structure can make the nitride layers improved incrystallinity and then make the nitride layers decreased in resistivity,resulting in smaller resistance of the n-type region and higher powerefficiency of the device.

In a preferred embodiment of the first and second nitride semiconductordevices, the super lattice layer is made by laminating first layerswhich is made of a nitride semiconductor and has a thickness of not morethan 100 angstroms and second layers which is made of a nitridesemiconductor having different constitutions from the first layer andhas a thickness of not more than 100 angstroms.

In order to keep or confine carriers in the active layer, at least oneof the first and second layers is preferably made of a nitridesemiconductor containing Al, especially Al_(Y)Ga_(1-Y)N (0<Y≦1).

In a second preferred embodiment of the first and second nitridesemiconductor devices, for the super lattice, the first layer ispreferably made of a nitride semiconductor represented by the formulaIn_(X)Ga_(1-X)N (0≦X≦1) and the second layer is preferably made of anitride semiconductor represented by the formula Al_(Y)Ga_(1-Y)N (0≦Y≦1,X=Y≠0). According to the second embodiment, all the nitride layers havea good crystallinity, which results in improving output of the nitridesemiconductor device (improvement of power efficiency). In LED or LDdevices, the forward voltage (hereinafter referred to Vf) and also thethreshold current and voltage can be lowered. In order to form a nitridelayer having better crystallinity in the first and second semiconductordevice, it is further recommendable that first layers of the superlattice structure are made of a nitride semiconductor represented by theformula In_(X)Ga_(1-X)N (0≦X<1) and said second layer is made of anitride semiconductor represented by the formula Al_(Y)Ga_(1-Y)N(0≦Y≦1).

In the above first and second semiconductor devices, it is preferablethat the first layer and the second layer are made of a nitridesemiconductor and have a thickness of not more than 70, especially 40angstroms, respectively, while said first layer and said second layerhave a thickness of not less than 10, especially 5 angstroms,respectively. The thickness within the above range makes it easy to formAl_(x)Ga_(1-y) (0<Y≦1), which layer is otherwise difficult to be formedwith a good crystallinity. Especially, in case that the super latticelayer can be made as at least one layer of the p-type region between thep-electrode and the active layer and also as at least one layer of then-type semiconductor region between the n-contact layer for currentcharging and the active layer, it is recommendable to get better effectthat thickness of the first and second layer should be set within theabove range.

In the above embodiment of the first and second nitride semiconductordevices, the p-type region is preferably provided with a p-side contactlayer having a thickness of not more than 500 angstroms, on which thep-electrode is to be formed. More preferably, the p-side contact layerhas a thickness of not more than 300 angstroms and not less than 10angstroms.

In the second nitride semiconductor device of the present invention,wherein the p-type region is provided with a p-side contact layer onwhich the p-electrode is to be formed, the super lattice layer ispreferably formed between the active layer and the p-side contact layer.

Further, in the second nitride semiconductor device of the presentinvention, the n-type region may comprise a second buffer layer made ofa nitride semiconductor which has a thickness of not less than 0.1 μmvia a first buffer layer on the substrate, an n-side contact layer madeof a nitride semiconductor doped with an n-type impurity on said secondbuffer layer, and an n-electrode being formed on the n-side contactlayer. This construction makes the n-side contact layer have highercarrier concentration and good crystallinity. In order to make then-side contact layer have much better crystallinity, it is preferablethat the concentration of the impurity in second buffer layer is lowerthan that in said n-side contact layer. Further, it is preferable thatat least one of the first and second buffer layers is a super latticelayer made by laminating nitride semiconductor layers of differentconstitutions with a thickness of not more than 100 angstroms in orderto make a nitride layer formed on the buffer layer and have a goodcrystallinity.

In the second nitride semiconductor device, wherein the n-type regionhas a n-side contact layer on which a n-electrode is to be formed, thesuper lattice layer is preferably formed between the active layer andthe n-side contact layer.

In the LD device, the layer formed between the active layer and then-side contact layer or between the active layer and the p-side contactlayer may be a cladding layer acting as a carrier keeping layer or alight guide layer, which is preferably made of the super latticestructure. Thereby, the super lattice structure can remarkably decreasethe threshold current and voltage. Especially, if the p-cladding layerbetween the active layer and the p-side contact layer, the p-claddinglayer of the super lattice structure is advantageous to lower thethreshold current and voltage. In the second nitride semiconductordevice of the present invention, it is preferable that at least one ofsaid first layer and said second layer is doped with an impurity whichmakes the conductivity of the layer n-type or p-type and the impurityconcentration doped to the first layer and the second layer to make theconductivity of the layers n-type or p-type, are different from eachother. The impurity for making the conductivity of the layer includesn-impurities belonging to IV-A, IV-B, VI-A and VI-B groups andp-impurities belonging to I-A, I-B, II-A, II-B groups (hereinafterreferred to n-impurity and p-impurity).

In the second nitride semiconductor device of the present invention, thesuper lattice layer may be formed as the n-side contact layer on whichthe n-electrode is to be formed, whereby the resistance of n-sidecontact layer can be lowered, resulting in further decreasing of thethreshold current and voltage in LD devices.

In the LD devices provided with the first or second nitridesemiconductor device of the present invention, if the laser device has asuper lattice layer in the p-type region, a ridge portion may be formedon the supper lattice layer and on the layer located over said superlattice layer in a manner that the longitudinal direction of the ridgeportion coincides with the direction of resonance and the ridge has apredetermined width.

In a preferred first laser diode of the present invention, whichcomprises an active layer in which laser is emitted between the n-typeregion including a n-side cladding layer and the p-type region includinga p-side cladding layer, the n-side cladding layer may be a superlattice layer made by laminating first layers made of a nitridesemiconductor having a thickness of not more than 100 angstroms andsecond layers made of a nitride semiconductor of a differentconstitution from the first layer and having a thickness of not morethan 100 angstroms, and said p-side cladding layer may be a superlattice layer made by laminating a third layer made of a nitridesemiconductor having a thickness of not more than 100 angstroms and afourth layer made of a nitride semiconductor of a different constitutionfrom the third layer and having a thickness of not more than 100angstroms. Due to this, during laser emission the threshold current andvoltage can be lowered. In this case, the ridge portion may be formed onsaid p-side cladding layer and on the layer located over said p-sidecladding layer in a manner that the longitudinal direction of the ridgecoincidences with the direction of resonance and the ridge has a desiredwidth.

According to a third aspect of the present invention, there is provideda third nitride semiconductor device which comprises an active layermade of a nitride semiconductor between a n-type region of one or morenitride semiconductor layers and a p-type region of one or more nitridesemiconductor layers, wherein at least one nitride semiconductor layerin the n-type region is a n-side super lattice made by laminating firstand second nitride semiconductor layers which have differentconstitutions and different concentrations of a n-type impurity fromeach other. Due to this construction, the nitride semiconductor layermade of the super lattice structure makes the electrical resistancethereof smaller and thus the total resistance of the n-type region canbe smaller.

According to a fourth aspect of the present invention, there is provideda nitride semiconductor device comprising an active layer made of anitride semiconductor between the n-type region of one or more nitridesemiconductor layers and the p-type region of one or more nitridesemiconductor layers, characterized in that at least one nitridesemiconductor layer in the p-type region is a p-side super lattice madeby laminating third and fourth nitride semiconductor layers which havedifferent constitutions and different concentrations of a p-typeimpurity from each other. The super lattice structure can make thenitride semiconductor layer comprising the super lattice structure havea lower resistance and then total resistance of the p-type region can bedecreased.

Please note that the first and second and the third and fourth of layersdoes not mean the laminating order in the specification.

According to a fifth aspect of the present invention, there is provideda nitride semiconductor device comprising an active layer made of anitride semiconductor between the n-type region of one or more nitridesemiconductor layers and the p-type semiconductor region of one or morenitride semiconductor layers, characterized in that at least one nitridesemiconductor layer in the n-type region is a n-side super lattice madeby laminating the first and second nitride semiconductor layers whichhave different constitutions and different concentrations of a n-typeimpurity from each other, and at least one nitride semiconductor layerin p-type region is a p-side super lattice made by laminating the thirdand fourth nitride semiconductor layers which have differentconstitutions and different concentrations of a p-type impurity fromeach other. The super lattice structure can make the resistance of thenitride semiconductor comprising super lattice structure smaller andthus total resistance of the p-type region can be decreased.

In a case that the third and fifth semiconductor devices are devices ofoptoelectronics such as light emitting devices and light receivingdevices, the n-side super lattice layer may be formed as at least one ofthe group consisting of a buffer layer formed on the substrate, a n-sidecontact layer for n-electrode, n-side cladding layer for confining orkeeping carriers and n-side light guide layer for guiding emission fromthe active layer. On the other hand, in the fourth and fifthsemiconductor device, the p-side superlattice layer may be formed as atleast one selected from the group consisting of the p-side contactlayer, the p-side cladding layer for confining carriers and the p-sidewave guide layer for guiding emission from the active layer.

In the third and fifth semiconductor devices of the present invention,for the n-side super lattice layer, the first nitride semiconductorlayer having a higher band gap may have a larger or smallerconcentration of the n-type impurity than the second nitridesemiconductor layer having a lower band gap. The larger impurityconcentration of the first layer than that of the second layer makescarrier generate in the first layer having a higher band gap and thenthe carrier injected into the second layer having a lower band gap tomove the carrier through the second layer having a smaller impurityconcentration and a larger mobility. Therefore, this construction makesthe n-side super lattice layer decreased in electrical resistance.

In a case that the impurity concentration of the first layer isrelatively larger than that of the second layer, the first layer of thesuper lattice layer in the first semiconductor device may decrease the nor p-impurity concentration at a part close to the second layercomparing with that at a part remote from the second layer, whichprevents the carrier moving through the second layer from scattering bythe impurity at the part close to the second layer, resulting inincrease of mobility of the second layer and thus lowering of theresistance of the super lattice layer.

In the embodiment of the third and fifth nitride semiconductor devices,if the n-impurity concentration in the first layer having a higher bandgap becomes larger, it is preferable that the n-impurity concentrationin the first layer ranges between 1×10¹⁷/cm³ and 1×10²⁰/cm³ and then-impurity concentration in the second layer is smaller than that of thefirst layer and not more than 1×10¹⁹/cm³. The n-impurity concentrationin the second layer having a smaller band gap is preferably not morethan 1×10¹⁸/cm³, more preferably not more than 1×10¹⁷/cm³. From theaspect of increasing the mobility of the second layer, a smallern-impurity concentration is better and an undoped layer or intentionallynot doped layer is most preferable.

If the impurity concentration of the first layer is smaller than that ofthe second layer, it is preferable that the n-impurity concentration ofthe second layer is smaller at a part close to the first layer than thatat a part remote from the first layer. For example, it is preferablethat the n-impurity concentration in the first layer is not more than1×10¹⁹/cm³ and the n-impurity in the second layer ranges between1×10¹⁷/cm³ and 1×10²⁰/cm³. The n-impurity concentration in the firstlayer having a smaller band gap is preferably not more than 1×10¹⁸/cm³,more preferably not more than 1×10¹⁷/cm³. The most preferable firstlayer is an undoped layer or intentionally not doped layer.

In order to form an n-side super lattice layer having a goodcrystallinity in the third and fifth semiconductor device, the firstnitride semiconductor layer may be made of Al_(Y)Ga_(1-Y)N (0<Y<1)capable of forming a relatively higher band gap layer having a goodcrystallinity and the second nitride semiconductor layer may be made ofIn_(X)Ga_(1-X)N (0≦X<1), capable of forming a relatively smaller bandgap layer having a good crystallinity.

The best second layer of the super lattice layer in the third and fifthsemiconductor devices, is a GaN layer. This construction is advantageousin manufacturing the super lattice layer because the same atmosphere canbe used to form the first layer (Al_(Y)Ga_(1-Y)N) and the second layer(GaN).

In the third and fifth nitride semiconductor devices, the first nitridesemiconductor layer may be made of Al_(X)Ga_(1-X)N (0<X<1) and thesecond nitride semiconductor layer may be made of Al_(Y)Ga_(1-Y)N(0<Y<1, X>Y). In this case, further, the first nitride semiconductorlayer or said second nitride semiconductor layer is preferably not dopedwith a n-type impurity.

In the fourth and fifth semiconductor devices of the present invention,for the p-side super lattice layer, the third nitride semiconductorlayer having a higher band gap may have a larger or smallerconcentration of the p-type impurity than that of the fourth nitridesemiconductor layer having a smaller band gap. The larger impurityconcentration of the third layer than that of the fourth layer makescarriers generate in the third layer having a higher band gap, and thecarriers injected into the fourth layer having a smaller band gap tomove the injected carriers through the fourth layer having a smallerimpurity concentration and a larger mobility, resulting in decreasingthe super lattice resistance.

Further, in the fourth and fifth semiconductor devices of the presentinvention, it is preferable that a part of the third nitridesemiconductor layer which is close to the fourth nitride semiconductorlayer has a lower concentration of the p-type impurity than a partremote or farther from the fourth nitride semiconductor layer, whichprevents the carrier moving through the fourth layer from scattering bythe impurity at the part close to the fourth layer, resulting inincrease of mobility of the fourth layer and thus further lowering ofthe resistance of the super lattice layer.

In the embodiment of the fourth and fifth nitride semiconductor devices,if the n-impurity concentration in the third layer becomes larger thanthat in the fourth layer, it is preferable that the n-impurityconcentration in the third layer having a larger band gap ranges between1×10¹⁸/cm³ and 1×10²¹/cm³ and the p-impurity concentration in the fourthlayer is smaller than that of the third layer and not more than1×10²⁰/cm³. The p-impurity concentration in the fourth layer having asmaller band gap is preferably not more than 1×10¹⁹/cm³, more preferablynot more than 1×10¹⁸/cm³. From the aspect of increasing the mobility ofthe second layer, a smaller n-impurity concentration is better and anundoped layer or intentionally not doped layer is most preferable.

In the fourth and fifth nitride semiconductor, if the impurityconcentration of the third layer is smaller than that of the fourthlayer, it is preferable that the p-impurity concentration of the fourthlayer is smaller at a part close to the third layer than that at a partremote from the third layer. For example, it is preferable that thep-impurity concentration in the first layer is not more than 1×10²⁰/cm³and the n-impurity in the second layer ranges between 1×10¹⁸/cm³ and1×10²¹/cm³. The n-impurity concentration in the third layer having asmaller band gap is preferably not more than 1×10¹⁹/cm³, more preferablynot more than 1×10¹⁸/cm³. The most preferable first layer is an undopedlayer or intentionally not doped layer.

In order to form a super lattice layer having a good crystallinity inthe fourth and fifth semiconductor device, the third nitridesemiconductor layer may be made of Al_(Y)Ga_(1-Y)N (0<Y<1) capable offorming a relatively higher band gap layer having a good crystallinityand the fourth nitride semiconductor layer may be made ofIn_(X)Ga_(1-X)N (0≦X<1). The best fourth layer of the super latticelayer in the third and fifth semiconductor devices, is a GaN layer. Thisconstruction is advantageous in manufacturing the super lattice layerbecause the same atmosphere can be used to form the third layer(Al_(Y)Ga_(1-Y)N) and the fourth layer (GaN).

In the fourth and fifth nitride semiconductor devices, the third nitridesemiconductor layer may be made of Al_(X)Ga_(1-X)N (0<X<1) and thefourth nitride semiconductor layer may be made of Al_(Y)Ga_(1-Y)N(0<Y<1, X>Y). In this case, further, the third nitride semiconductorlayer or the fourth second nitride semiconductor layer is preferably notdoped with a n-type impurity.

In the fifth nitride semiconductor, for the n-side super lattice layer,the first nitride semiconductor layer may be provided with a higher bandgap energy and a larger concentration of the n-type impurity than thesecond nitride semiconductor layer, and for the p-side super latticelayer, the third nitride semiconductor layer may be provided with ahigher band gap energy and a larger concentration of the p-type impuritythan the fourth nitride semiconductor layer. In this case, it ispreferable that the concentration of the n-type impurity in the firstnitride semiconductor layer ranges between 1×10¹⁷/cm³ and 1×10²⁰/cm³ andthe concentration of the n-type impurity in the second nitridesemiconductor layer is not more than 1×10¹⁹/cm³, and the concentrationof the p-type impurity in the third nitride semiconductor layer rangesbetween 1×10¹⁸/cm³ and 1×10²¹/cm³ and the concentration of the p-typeimpurity in the fourth nitride semiconductor layer is not more than1×10²⁰/cm³.

Further, in the fifth nitride semiconductor device, for the n-side superlattice layer, the first nitride semiconductor layer may be providedwith a higher band gap energy and a larger concentration of the n-typeimpurity than said second nitride semiconductor layer, and for thep-side super lattice layer, the third nitride semiconductor layer may beprovided with a higher band gap energy and a smaller concentration ofthe p-type impurity than the fourth nitride semiconductor layer. In thiscase, it is preferable that the concentration of the n-type impurity inthe first nitride semiconductor layer ranges between 1×10¹⁷/cm³ and1×10²⁰/cm³ and the concentration of the n-type impurity in the secondnitride semiconductor layer is not more than 1×10¹⁹/cm³, and theconcentration of the p-type impurity in the third nitride semiconductorlayer is not more than 1×10²⁰/cm³ and the concentration of the p-typeimpurity in the fourth nitride semiconductor layer ranges between1×10¹⁸/cm³ and 1×10²¹/cm³.

Furthermore, in the fifth nitride semiconductor device, for the n-sidesuper lattice layer, the first nitride semiconductor layer may bedesigned to have a higher band gap energy and a smaller concentration ofthe n-type impurity than the second nitride semiconductor layer, and forthe p-side super lattice layer, the third nitride semiconductor layermay be designed to have a higher band gap energy and a largerconcentration of the p-type impurity than the fourth nitridesemiconductor layer. In this case, it is preferable that theconcentration of the n-type impurity in the first nitride semiconductorlayer is not more than 1×10¹⁹/cm³ and the concentration of the n-typeimpurity in the second nitride semiconductor layer ranges between1×10¹⁷/cm³ and 1×10²⁰/cm³, and the concentration of the p-type impurityin the third nitride semiconductor layer ranges between 1×10¹⁸/cm³ and1×10²¹/cm³ and the concentration of the p-type impurity in the fourthnitride semiconductor layer is not more than 1×10²⁰/cm³.

Further, in the fifth nitride semiconductor device, for the n-side superlattice layer, the first nitride semiconductor layer may be designed tohave a higher band gap energy and a smaller concentration of the n-typeimpurity than the second nitride semiconductor layer, and for the p-sidesuper lattice layer, the third nitride semiconductor layer may bedesigned to have a higher band gap energy and a smaller concentration ofthe p-type impurity than the fourth nitride semiconductor layer. In thiscase it is preferable that the concentration of the n-type impurity inthe first nitride semiconductor layer is not more than 1×10¹⁹/cm³ andthe concentration of the n-type impurity in the second nitridesemiconductor layer ranges between 1×10¹⁷/cm³ and 1×10²⁰/cm³, and theconcentration of the p-type impurity in the third nitride semiconductorlayer is not more than 1×10²⁰/cm³ and the concentration of the p-typeimpurity in the fourth nitride semiconductor layer ranges between1×10¹⁸/cm³˜1×10²¹/cm³.

Furthermore, in the fifth nitride semiconductor device, for the n-sidesuper lattice layer, the first nitride semiconductor layer may be madeof Al_(Y)Ga_(1-Y)N (0<Y<1) and the second nitride semiconductor layermay be made of In_(X)Ga_(1-X)N (0≦X<1), and for the p-side super latticelayer, the third nitride semiconductor layer may be made ofAl_(Y)Ga_(1-Y)N (0<Y<1) and the fourth nitride semiconductor layer maybe made of In_(X)Ga_(1-X)N (0≦X<1). In this case, it is preferable thatthe second and fourth nitride semiconductor layers are made of GaN,respectively.

Further, in the fifth nitride semiconductor device, for the n-side superlattice layer, the first nitride semiconductor layer may be made ofAl_(X)Ga_(1-X)N (0<X<1) and the second nitride semiconductor layer ismade of Al_(Y)Ga_(1-Y)N (0<Y<1, X>Y), and for the p-side super latticelayer, the third nitride semiconductor layer may be made ofAl_(X)Ga_(1-X)N (0<X<1) and the fourth nitride semiconductor layer maybe made of Al_(Y)Ga_(1-Y)N (0≦Y≦1, X>Y).

Furthermore, in the fifth nitride semiconductor device, it is preferablethat the first nitride semiconductor layer or the second nitridesemiconductor layer is an undoped layer to which a n-type impurity isnot doped. It is also preferable that the third nitride semiconductorlayer or the fourth nitride semiconductor layer is an undoped layerwhich is not doped with a p-type impurity.

In the third, fourth and fifth nitride semiconductor device, the activelayer preferably includes a InGaN layer. The InGaN layer in the activelayer is preferably in a form of a quantum well layer. The active layermay be SQW or MQW.

According to the present invention, there is provided a second nitridesemiconductor LD device comprising an active layer between a p-sidecladding layer and a n-side cladding layer, and

at least one of the p-side and the n-side cladding layers is the n-sidesuper lattice layer or the p-side super lattice layer respectively. TheLD device can operate at a lower threshold current. In the second LDdevice, it is preferable that an optical wave guide layer made of anitride semiconductor containing In or GaN which has an impurityconcentration of not more than 1×10¹⁹/cm³, the optical wave guide layerbeing formed at least either between the p-side cladding layer and theactive layer or between the p-side cladding layer and the active layer.In this case, the wave guide can prevent the emission generated fromdisappearing due to a low absorption rate of the optical wave guide,which causes a LD device capable of waving at a low gain. In this case,in order to further decrease the light absorption rate, it is morepreferable that the impurity concentration of the wave guide layer isnot more than 1×10¹⁸/cm³, especially not more than 1×10¹⁷/cm³. The mostpreferable layer is an undoped one. The optical wave guide layer may bemade of the super lattice structure.

Furthermore, it is recommendable that there is provided with a cap layermade of a nitride semiconductor between the optical wave guide layer andthe active layer. It is preferable that the cap layer having a higherband gap energy than the well layer in the active layer and also theoptical wave guide layer and having a thickness of not more than 0.1 μmis formed between said optical wave guide layer and said active layer.It is more preferable that the cap layer contains an impurity of notless than 1×10¹⁸/cm³. The cap layer can make a leak current loweredbecause of a higher band gap. It is effective that the optical waveguide layer and cap layer are formed in the p-type region or thesemiconductor region of p-conductivity side.

The third to the fifth nitride semiconductor devices of the presentinvention may be preferably formed on a nitride semiconductor substrate.The nitride semiconductor substrate can be prepared by a method ofgrowing a nitride semiconductor layer on an auxiliary substrate made ofa material other than nitride semiconductor, forming a protective filmon the grown nitride semiconductor layer so as to expose partially thesurface thereof, thereafter growing a nitride semiconductor layer tocover the protective film from the exposed nitride semiconductor layer.The nitride semiconductor substrate can make it better the crystallinityof every layers in the third to the fifth nitride semiconductor device.In this case, the auxiliary substrate and the protective film can beremoved from the nitride semiconductor substrate before or after thedevice layers are formed on the nitride semiconductor substrate. The caplayer had better be formed in the p-type region.

In a preferred embodiment of the LD device according to the presentinvention, wherein p-side cladding layer is a super lattice layer, it ispreferable that a ridge portion is formed on the p-side cladding layerand on the layer located over the p-side cladding layer in a manner thatthe longitudinal direction of the ridge portion coincides with thedirection of resonance and the ridge has a predetermined width.

According to a sixth aspect of the present invention, there is provideda nitride semiconductor light emitting device comprising an active layerincluding a first nitride semiconductor layer containing In between an-side cladding layer and a p-side cladding layer, characterized in thatthe n-side cladding layer is a super lattice layer comprising a secondnitride semiconductor layer containing Al and has a total thickness ofnot less than 0.5 μm and an average composition of Al in said n-sidecladding layer is set in a way that the product of said average Alcomposition in % contained in said n-side cladding layer multiplied bythe thickness in μm of said n-side cladding layer is not less than 4.4.This causes the optical confinement effect by the n-side cladding layerimproved, resulting in a long lifetime and a high responsibility of theLD device due to a lower wave oscillation threshold.

In an embodiment of the LD device formed on the substrate, wherein then-side cladding layer is usually formed at a part close to the substratein the n-type region, if the confinement effect of the light is notsufficient, the light leaked through the n-side cladding layer isreflected by the substrate, resulting in disturbing shapes of far andnear field pattern such as observation of multi-spots of laser beam.However, the n-side cladding layer in the sixth nitride semiconductordevice, makes the light confinement effect improved, which prevent thenear and far field patterns from being disturbed, that is, which canmake a single laser spot.

In a preferred embodiment of the sixth nitride semiconductor device ofthe present invention, the n-side cladding layer has a thickness of notless than 0.8 μm and an average Al composition of not less than 5.5%. Ina more preferable embodiment, the n-side cladding layer has a thicknessof not less than 1.0 μm and an average Al composition of not less than5%. In a most preferable embodiment, the n-side cladding layer has athickness of not less than 1.2 μm and an average Al composition of notless than 4.5%.

In the sixth nitride semiconductor device, it is preferable that thep-side cladding layer is a super lattice layer comprising a thirdnitride semiconductor layer containing Al and has a thickness smallerthan said n-side cladding layer. More preferably, the p-side claddinglayer has a thickness of less than 1.0 μm and the thickness of then-side cladding layer and said p-side cladding layer including saidactive layer is set to range between 200 angstroms and 1.0 μm.

BRIEF EXPLANATION OF THE DRAWING

FIG. 1 is a schematic sectional view of the nitride semiconductor device(LED) structure of the first embodiment according to the presentinvention,

FIG. 2 is a schematic sectional view of the nitride semiconductor device(LD) structure of the second embodiment according to the presentinvention,

FIG. 3 is a schematic sectional view of the nitride semiconductor device(LD) structure of the third embodiment according to the presentinvention,

FIG. 4 is a perspective view of the LD shown in FIG. 5,

FIG. 5 is a graph showing the relation between the thickness of thep-side contact layer and the threshold voltage in Example 1 of thepresent invention,

FIG. 6 is a schematic sectional view of the LD in Example 26 of thepresent invention,

FIG. 7 is a schematic sectional view of the LD in Example 28 of thepresent invention,

FIG. 8 is a schematic sectional view of the LD in Example 33 of thepresent invention,

FIG. 9 is a schematic view of the nitride semiconductor device (LD)structure of the fourth embodiment according to the present invention.

PREFERRED EMBODIMENT OF THE INVENTION

Preferred embodiments of the nitride semiconductor device according tothe present invention will now be described below with reference to theaccompanying drawings.

Embodiment 1

FIG. 1 is a schematic cross sectional view showing the configuration ofthe nitride semiconductor device according to the first embodiment ofthe present invention. The nitride semiconductor device is an LED devicehaving such a basic configuration as a buffer layer 2 made of GaN, ann-side contact layer 3 made of Si-doped n-type GaN, an active layer 4made of InGaN of single quantum well structure, a p-side cladding layer5 made of a super lattice layer comprising a first layer and a secondlayer of different constitutions being laminated, and a p-type contactlayer 6 made of Mg-doped GaN are laminated successively on a substrate 1made of sapphire. In the nitride semiconductor device of the firstembodiment, nearly entire surface of the p-side contact layer 6 iscovered with a planar electrode 7 formed thereon which is provided witha p electrode 8 for the purpose of bonding installed on the surfacethereof, while an n electrode 9 is installed on the surface of then-side contact layer 2 which is exposed by removing a part of nitridesemiconductor layer by etching. The planar electrode 7 allows light totransmit.

The nitride semiconductor device of the first embodiment has the p-typecladding layer 5 having a low resistance and comprising a super latticelayer made by laminating the first layer which is made ofAl_(Y)Ga_(1-Y)N (0≦Y≦1) doped with Mg as a p-type impurity, for example,and has a thickness of 30 angstroms, and a second layer which is made ofp-type Al_(Y)Ga_(1-Y)N (0≦Y≦1) doped with the same amount of Mg as inthe first layer as the p-type impurity and has a thickness of 30angstroms, and therefore Vf can be made lower. When the super latticelayer is formed on the p layer side as described above, the superlattice layer is rendered p-type conductivity by doping the first layerand/or the second layer with a p-type impurity such as Mg, Zn, Cd, Be,etc. The first layer and the second layer may be laminated either in theorder of 1st, 2nd, 1st and so on, or in the order of 2nd, 1st, 2nd andso on, provided that at least two layers are laminated.

The first layer and the second layer which is made of the nitridesemiconductor and constitute the super lattice layer are not limited tothe layer made of In_(X)Ga_(1-X)N (0≦X≦1) and the layer made ofAl_(Y)Ga_(1-Y)N (0≦Y≦1), and may be other layers provided that they aremade of nitride semiconductors of different constitutions. The firstlayer and the second layer may or may not have the same band gap energy.For example, when the first layer is made of In_(X)Ga_(1-X)N (0≦Y≦1) andthe second layer is made of Al_(Y)Ga_(1-Y)N (0<Y≦1), band gap energy ofthe second layer becomes higher than that of the first layer, althoughwhen the first layer is made of In_(X)Ga_(1-X)N (0≦X≦1) and the secondlayer is made of In_(Z)Al_(1-Z)N (0≦z≦1), the first layer and the secondlayer may be of different constitutions but have the same band gapenergy. Also when the first layer is made of Al_(Y)Ga_(1-Y)N (0≦Y≦1) andthe second layer is made of In_(Z)Al_(1-Z)N (0<z≦1), the first layer andthe second layer may be of different constitutions but have the sameband gap energy.

That is, according to the present invention, the first layer and thesecond layer may or may not have the same band gap energy, provided thatthey constitute a super lattice layer which has a function to bedescribed later. As described above, the super lattice layer referred toherein is a layer made by laminating extremely thin layers of differentconstitutions, which is free from lattice defects accompanying latticemismatch, because every layer is very thin, having broad implicationsincluding the quantum well structure. Although the super lattice layerdoes not have inner defects, it usually has a strain accompanyinglattice mismatch, and is hence called also a stained layer superlattice. According to the present invention, the first layer and thesecond layer remain to be nitride semiconductors as long as they includeN even when part of N (nitrogen) atoms are substituted with V groupelements such as As and P.

According to the present invention, because the thickness of the firstlayer and the second layer which constitute the super lattice layerreaches the elastic strain limit or greater when it is greater than 100angstroms, and microscopic cracks or crystal defects are likely to begenerated in the film, and therefore the thickness is preferably set towithin 100 angstroms. Lower limit of the thickness of the first layerand the second layer is not limited and may be of any value providedthat it is monoatomic layer or thicker. According to the firstembodiment, the first layer and the second layer are preferably 70angstroms or thinner to obtain better quality of crystal and, morepreferably, further thinner in a range from 10 angstroms to 40angstroms. According to the present invention, although the thicknessmay be 10 angstroms or less (for example, monoatomic layer or diatomiclayer), setting the thickness within 10 angstroms leads to lamination oftoo many layers in case a cladding layer having the thickness of 500angstroms or greater is formed in super lattice structure, for example,resulting in longer forming time and increased labor in themanufacturing process. Thus the first layer and the second layer arepreferably set to be thicker than 10 angstroms.

In the case of the nitride semiconductor device of the first embodimentshown in FIG. 1, the p-type cladding layer 5 made of super lattice layeris formed between the active layer 4 and the p-side contact layer 6which is a current injection layer, and functions as a carrier trappinglayer. When a super lattice layer is used as a carrier trapping layer inthis way, mean band gap energy of the super lattice layer must be higherthan that of the active layer. In a nitride semiconductor device,therefore, a nitride semiconductor which includes Al in such a form asAlN, AlGaN or InAlN having relatively high band gap energy is used as acarrier trapping layer. Among such layers, AlGaN has a tendency todevelop cracks during crystal growing process when grown to be thick asa single layer.

In the first embodiment, therefore, a super lattice layer having lesscracks with excellent quality of crystal is formed and used as acladding layer having a high band gap energy, by making at least one ofthe first layer and the second layer from a nitride semiconductor whichincludes at least Al, preferably Al_(Y)Ga_(1-Y)N (0<Y≦1), and growingthe first layer and the second layer alternately with the thicknesswithin the elastic strain limit.

In this case, when a nitride semiconductor layer which does not includeAl is grown as the first layer to the thickness within 100 angstroms andthe second layer made of a nitride semiconductor which includes Al isgrown thereon, the first layer also functions as a buffer layer whengrowing the second layer so that cracks are less likely to develop inthe second layer, making it possible to form a super lattice layer whichhas less cracks with excellent quality of crystal. Thus according to thefirst embodiment, it is desirable that the super lattice layer be formedfrom the first layer (the second layer) made of In_(X)Ga_(1-X)N (0≦X≦1)and the second layer (the first layer) made of Al_(Y)Ga_(1-Y)N (0≦Y≦1,X≠Y=0).

Also in the nitride semiconductor device of the first embodiment, atleast one of the first layer and the second layer which constitute thep-side cladding layer 5, that is the super lattice layer, is preferablydoped with a p-type impurity which makes the conductivity of the layerp-type, for the purpose of adjusting the carrier concentration. In casethe first layer and the second layer are doped with a p-type impurity,the first layer and the second layer may be doped in differentconcentrations. This is because, when the first layer and the secondlayer are doped in different concentrations, carrier concentration inone of the layers becomes substantially higher, thus making it possibleto decrease the resistance of the super lattice layer as a whole. Thusaccording to the present invention, the first layer and the second layermay be doped in different concentrations, or alternatively, only one ofthe first layer and the second layer may be doped.

Concentrations of the impurity doped in the first layer and the secondlayer are preferably controlled in a range from 1×10¹⁶/cm³ to1×10²²/cm³, more preferably from 1×10¹⁷/cm³ to 1×10²¹/cm³, and mostpreferably from 1×10¹⁸/cm³ to 2×10²⁰/cm³ in the case of p type impurity,although the present invention is not limited to this configuration.This is because, when impurity concentration is lower than 1×10¹⁶/cm³,it becomes difficult to obtain the effect of reducing Vf and thethreshold voltage and, when impurity concentration is higher than1_(—)10²²/cm³, quality of crystal of the super lattice layerdeteriorate. Concentration of n type impurity is also desired to becontrolled within a similar range, for the same reason.

The super lattice layer constituted as described above is formed bylaminating the first layer and the second layer alternately with thethickness within the elastic strain limit, and therefore lattice defectsof the crystal can be reduced and microscopic cracks can be reduced,thus improving the quality of crystal drastically. As a result, theamount of impurity doped can be increased thereby increasing the carrierconcentrations in the n-type nitride semiconductor layer and in thep-type nitride semiconductor layer without substantially deterioratingthe quality of crystal, thus allowing the carrier to move without beingscattered by crystal defects, and therefore it is made possible toreduce the resistivity at least one order of magnitude lower than thatof the p-type or n-type nitride semiconductor which does not have superlattice structure.

Thus in the nitride semiconductor device (LED device) of the firstembodiment, Vf can be reduced by forming the p-type cladding layer 5, ofa semiconductor region 251 of p conductivity side (the region comprisingthe p-type cladding layer 5 and the p-type contact layer 6 in the firstembodiment) where it has been difficult to obtain a nitridesemiconductor layer having a low resistance in the prior art, by usingthe super lattice layer, thereby reducing the resistance of the p-typecladding layer 5. That is, nitride semiconductor is a semiconductorwhich is difficult to obtain in the form of p-type crystal and, if everobtained, resistivity is usually at least two orders of magnitude highercompared to n-type nitride semiconductor. Therefore, when a superlattice layer of type p is formed on p-type conductivity side,resistance of the p-type layer constituted from the super lattice layercan be reduced to an extremely low level, resulting in a remarkabledecrease in Vf. As a prior art technology to obtain a p-type crystal,such a process has been known where a nitride semiconductor layer dopedwith a p-type impurity is annealed and hydrogen is removed therebymanufacturing a p-type nitride semiconductor (Japanese Patent No.2540791). However, the p-type nitride semiconductor thus obtained hasresistivity as high as several ohm-centimeters at the lowest. By turningthis p-type layer into p-type super lattice layer, better quality ofcrystal can be obtained. According to the study of the presentinventors, resistivity of the p-type layer can be reduced to a level atleast one order of magnitude lower than that of the prior art, resultingin a conspicuous effect of decreasing Vf.

According to the first embodiment, a super lattice layer which has goodquality of crystal and is free from cracks can be formed by constitutingthe first layer (the second layer) made from In_(X)Ga_(1-X)N (0≦x≦1) andthe second layer (the first layer) made from Al_(Y)Ga_(1-Y)N (0≦Y≦1,X≠Y=0) as described above, and therefore service life of the device canbe elongated.

Now the present invention will be compared with examples of the priorart disclosed in literature known to the public, including JapanesePatent Publication filed by the present inventors in the past.

As a technology similar to the present invention, the present inventorsproposed the technology disclosed in Japanese Patent Non-examined PatentPublication No. 8-228048. This technology forms a multi-layer film madeof AlGaN, GaN, InGaN, etc. as a laser beam reflecting film on theoutside of an n-type cladding layer and/or the outside (on the sidefarther from an active layer) of a p-type cladding layer which interposethe active layer. According to this technology, because the multi-layerfilm is formed as the light reflecting film, each layer is designed tohave a thickness of λ/4n (n is a refractive index of the nitridesemiconductor and λ is a wavelength) and becomes very thick. Thus eachlayer of the multi-layer film has the thickness not within the elasticstrain limit. U.S. Pat. No. 5,146,465 discloses a laser device havingsuch a constitution as an active layer is interposed between mirrorsmade of Al_(X)Ga_(1-X)N/Al_(Y)Ga_(1-Y)N. This technology, similarly tothat described above, makes AlGaN/AlGaN act as mirrors, and thereforerequires each layer to be thick. Particularly it is very difficult tolaminate many layers of hard semiconductor such as AlGaN withoutgenerating cracks.

According to the first embodiment, unlike the technologies describedabove, the first and the second layers are set to have such thethickness (preferably within 100 angstroms, namely within the criticalfilm thickness for both layers) so as to constitute a super latticelayer. That is, the present invention utilizes the effect of strainedlayer super lattice of a nitride semiconductor which constitutes thesuper lattice layer, thereby to improve the quality of crystal andreduce Vf.

Japanese Non-examined Patent Publication Nos. 5′-110138 and 5-110139disclose a method to obtain a crystal of Al_(Y)Ga_(1-Y)N by laminatingthin films of AlN and GaN. This technology is to laminate AlN and GaNlayers which are several tens of angstroms thick thereby to obtain amixed crystal of Al_(Y)Ga_(1-Y)N having a specified mix proportion, andis different from the technology of this invention. And because thetechnology does not include an active layer made of InGaN, the superlattice layer is liable to cracks developing therein. JapaneseNon-examined Patent Publication Nos. 6-21511 and No. 6-268257 alsodisclose a light emitting device of double-hetero structure having anactive layer of multiple quantum well structure made by laminating GaNand InGaN or InGaN and InGaN. This technology is also different from thepresent invention which proposes a technology of making a multi-layerstructure other than the active layer. Japanese Non-examined PatentPublication No. 2-288371 (U.S. Pat. No. 5,005,057) discloses a structurehaving a super lattice layer other than the active later. However, thesupper lattice disclosed in Japanese Non-examined Patent Publication No.2-288371 consist of BP layers and GaAlN layers while the supper latticeof present invention consist of nitride semiconductor layers havingdifferent constitution each other.

Therefore, this technology is different from the present invention withregard to structure and an effect.

Further in the device of the present invention, the effect of the superlattice layer becomes conspicuous when the active layer is provided witha nitride semiconductor, which includes at least indium, such as InGaN.InGaN active layer has less band gap energy and is most suitable for theactive layer of a nitride semiconductor device. Therefore, when superlattice layers comprising In_(X)Ga_(1-X)N and Al_(Y)Ga_(1-Y)N are formedto interpose the active layer, difference in the band gap energy anddifference in refractive index from those of the active layer can beincreased, thereby making the super lattice layer capable of functioningas an excellent light trapping layer when forming a laser device(applied to the nitride semiconductor device of second embodiment).Moreover, because InGaN has quality of crystal of being softer thanother nitride semiconductors which include Al such as AlGaN, use ofInGaN as the active layer makes the laminated nitride semiconductorlayers less liable to cracks. Conversely, use of a nitride semiconductorsuch as AlGaN as the active layer makes it likely that cracks develop inthe entire crystal because the crystal is hard in nature.

According to the first embodiment, it is desirable that the thickness ofthe p-side contact layer 6 be controlled to within 500 angstroms, morepreferably within 300 angstroms and most preferably within 200angstroms. This is because resistivity can be further decreased bycontrolling the thickness of the p-type nitride semiconductor which hasa high resistivity of several ohm-centimeters or higher within 500angstroms, thus reducing the threshold current and voltage. It is alsomade possible to increase the amount of hydrogen removed from the p-typelayer, thereby further reducing the resistivity.

As described above in detail, in the nitride semiconductor device (LEDdevice) of the first embodiment, the p-type cladding layer 5 isconstituted from the super lattice layer made by laminating the firstlayer and the second layer, and therefore resistance of the p-typecladding layer 5 can be made extremely low and the forward voltage Vf ofthe LED device can be reduced.

Although the first embodiment uses the super lattice layer in the p-sidecladding layer 5, the present invention is not limited to thisconfiguration and a p-type super lattice layer may also be used in thep-type contact layer 6. That is, the p-type contact layer 6 to whichcurrent (positive holes) is injected may also be made as p-type superlattice layer formed by laminating the first layer made ofIn_(X)Ga_(1-X)N and the second layer made of Al_(Y)Ga_(1-Y)N. When thep-type contact layer 6 is made as a super lattice layer and the band gapenergy of the first layer is less than that of the second layer, it ispreferable that the first layer made of In_(X)Ga_(1-X)N or GaN having alow band gap energy be placed on the top and put into contact with the pelectrode, so that the contact resistance with the p electrode becomeslower thereby providing better ohmic contact. This is because the firstlayer which has lower band gap energy has a tendency to provide anitride semiconductor layer of higher carrier concentration than in thecase of the second layer. Also according to the present invention, whena p-type nitride semiconductor layer other than the p-side claddinglayer and the p-side contact layer is further formed on thesemiconductor region 251 of p conductivity side, the p-type nitridesemiconductor layer may be constituted from a super lattice layer.

Although the first embodiment uses the super lattice layer in the p-sidecladding layer 5, the present invention is not limited to theconfiguration of the semiconductor region 251 of p-type conductivity,and an n-type super lattice layer may also be used in the n-type contactlayer 3 of the semiconductor region 201 of n conductivity side. In sucha case as the n-type contact layer 3 is used as the super lattice layer,the first layer and/or the second layer can be doped with an n-typeimpurity such as Si and Ge, for example, thereby forming a super latticelayer having n-type conductivity as the n-type contact layer 3 betweenthe substrate 1 and the active layer 4. In this case, it was verifiedthat making the n-type contact layer 3 in the form of super latticelayer having different impurity concentration, in particular, decreasesthe resistance in the transverse direction and tends to decrease thethreshold voltage and current in a laser diode, for example.

This is supposed to be due to an effect similar to HEMT (High-ElectronMobility Transistor) as described below, in case a super lattice layer,which is made by doping a layer having higher band gap energy withgreater amount of n-type impurity, is formed as n-type contact layer. Inthe super lattice layer made by laminating the first layer (the secondlayer) which is doped with an n-type impurity and has a greater band gapand the second layer (the first layer) which is undoped (state of beingnot doped will be called undoped hereinafter) and has less band gap, thelayer having higher band gap energy is depleted in the interface betweenthe layer doped with the n-type impurity and the undoped layer, andelectrons (two dimensional electron gas) accumulate in the interfacearound the layer which has lower band gap energy and a thickness ofabout 100 angstroms. It is supposed that, because the two dimensionalelectron gas is generated in the layer having lower band gap energy, theelectrons move without being scattered by the impurity, and thereforeelectron mobility in the super lattice layer increases and resistivitydecreases.

Also according to the present invention, when an n-side cladding layeris installed on a semiconductor layer 201 of n conductivity side, then-side cladding layer may be made in super lattice layer. In case ann-type nitride semiconductor layer other than the n-side contact layerand the n-side cladding layer is installed in a semiconductor region 201of n conductivity side, the n-type nitride semiconductor layer may bemade in the form of super lattice layer. However, it is a matter ofcourse that, in case the nitride semiconductor layer comprising thesuper lattice layer is installed in the semiconductor region 201 of nconductivity side, it is desirable that either the n-type cladding layeracting as a carrier trapping layer or the n-type contact layer 3 towhich current (electrons) is injected be made in super latticestructure.

In case a super lattice layer is formed in the semiconductor region 201of n conductivity side placed between the active layer 4 and thesubstrate 1 as described above, the first layer and the second layerwhich constitute the super lattice layer may not be doped with impurity.This is because nitride semiconductor has a nature of becoming n-typeeven when undoped. It is desirable, however, to dope the first layer andthe second layer with n-type impurity such as Si and Ge to make adifference in the impurity concentration as described above, even whenforming on the n layer side.

When a super lattice layer is formed in the semiconductor region 201 ofn conductivity side, the effect thereof will be an improvement in thequality of crystal similarly to the case of forming the super latticelayer in the semiconductor region 251 of p-type conductivity side.Specifically, in the case of a nitride semiconductor device which hashetero junction, carrier trapping layers of n-type and p-type areusually constituted from AlGaN which has a band gap energy which ishigher than the active layer. Crystal of AlGaN is very difficult togrow, and cracks tend to develop in the crystal when a layer having thethickness of 0.5 μm or greater is grown with a single constitution.However, when a super lattice layer is made by laminating the firstlayer and the second layer each with a thickness within the elasticstrain limit, as in the case of the present invention, the first layerand the second layer can be laminated with good quality of crystal andtherefore a cladding layer having good quality of crystal can be grown.Thus because the nitride semiconductor is given good quality of crystalall over the semiconductor region 201 of n conductivity side, mobilitycan be increased throughout the semiconductor region 201 of nconductivity side and therefore Vf of a device wherein the super latticelayer is used as a cladding layer can be decreased. Further, when thesuper lattice layer is doped with an impurity such as Si and Ge and thesuper lattice layer is used as the contact layer, it is supposed thatthe effect similar to that of HEMT described previously appearsmarkedly, making it possible to decrease the threshold voltage and Vffurther.

Also according to the present invention, the super lattice layer may notbe doped with the impurity which determines the conductivity types ofthe first layer and the second layer. The super lattice layer which isnot doped with the impurity may be formed as any of the layers betweenthe active layer and the substrate, in case the layer is in thesemiconductor region 201 of n conductivity side, and may be formed asany of the layers between the carrier trapping layer (light trappinglayer) and the active layer, provided that the layer is in thesemiconductor region 251 of p conductivity side.

According to the present invention, as described above, because thesuper lattice layer is used either as the cladding layer acting as acarrier trapping layer formed in the semiconductor region 201 of nconductivity side or in the semiconductor region 251 of p conductivityside interposing the active layer, or as an optical waveguide layer ofthe active layer or as a current injection layer provided with anelectrode being formed in contact therewith, it is desirable that meanband gap energy of the nitride semiconductor constituting the superlattice layer be controlled to be higher than that of the active layer.

While the region comprising the nitride semiconductor layers placedbetween the active layer and the p electrode is referred to as thesemiconductor region on p conductivity side in this specification, thisdoes not mean that all the nitride semiconductor layers constituting thesemiconductor region have the p-type conductivity. Similarly, the regioncomprising nitride semiconductor layers between the active layer and aGaN substrate 100 is referred to as the semiconductor region of n-sideconductivity, this does not mean that all the nitride semiconductorlayers constituting the region have the p-type conductivity.

Embodiment 2

Now the second embodiment of the present invention will be describedbelow.

FIG. 2 is a schematic cross sectional view (cross section perpendicularto the direction of propagation of laser light) showing theconfiguration of a nitride semiconductor device according to the secondembodiment of the present invention. The nitride semiconductor deviceis, for example, a nitride semiconductor laser diode device which has anactive layer 16 comprising a nitride semiconductor interposed by asemiconductor region 202 of n conductivity side (consisting of an n-sidecontact layer 12, a crack preventing layer 13, an n-side cladding layer14 and an n-side optical waveguide layer 15) and a semiconductor region252 of p conductivity side (consisting of a cap layer 17, a p-sideoptical waveguide layer 18, a p-side cladding layer 19 and a p-sidecontact layer 20) provided on a C plane of a substrate 10 made ofsapphire or the like.

In the nitride semiconductor device of the second embodiment, thresholdvoltage of the nitride semiconductor device which is a laser diode isset to a low level by forming the n-side cladding layer 14 in thesemiconductor region 202 of n conductivity side in the form of superlattice layer and forming the p-side cladding layer 19 in thesemiconductor region 252 of p conductivity side in the form of superlattice layer. The nitride semiconductor device according to the secondembodiment of the present invention will be described in detail below byreferring to FIG. 2.

In the nitride semiconductor device of the second embodiment, first then-side contact layer 12 is formed on the substrate 10 via a buffer layer11 and a second buffer layer 112, then the crack preventing layer 13,the n-side cladding layer 14 and the n-side optical waveguide layer 15are laminated on the n-side contact layer 12, thereby to form thesemiconductor region 202 of n conductivity side. Formed on the surfacesof the n-side contact layer 12 exposed on both sides of the crackpreventing layer 13 are n-side electrodes 23 which make ohmic contactwith the n-side contact layer 12, while an n-side pad electrode for thepurpose of wire bonding, for example, is formed on the n-side electrode23. Then the active layer 16 made of a nitride semiconductor is formedon the n-side optical waveguide layer 15, and the cap layer 17, thep-side optical waveguide layer 18, the p-side cladding layer 19 and thep-side contact layer 20 are formed on the active layer 16, thereby toform the semiconductor region 252 of p conductivity side. Further on thep-side contact layer 20, the p-side electrode 21 which makes ohmiccontact with the p-side contact layer 20 is formed, and the p-side padelectrode for the purpose of wire bonding, for example, is formed on thep-side electrode 21. Formed on the p-side contact layer 20 and thep-side cladding layer 19 is a ridge which extends long in the directionof resonance, thereby to trap light in the active layer 16 in thetransverse direction (direction perpendicular to the direction ofpropagation) and form a resonator which resonates in the longitudinaldirection of the ridge by using a cleavage plane which is at rightangles to the ridge, thus making the laser oscillate.

Components of the nitride semiconductor device of the second embodimentwill now be described below.

(Substrate 10)

The substrate 10 may be made of, in addition to sapphire havingprincipal plane in C plane, sapphire having principal plane in R planeor A plane, insulating substrate such as spinel (MgAl₂O₄), or othersemiconductor substrate such as SiC (including 6H, 4H and 3C), ZnS, ZnO,GaAs and GaN.

(Buffer Layer 11)

The buffer layer 11 is formed by growing AlN, GaN, AlGaN, InGaN, etc.,for example, at a temperature within 900° C. to a thickness of severaltens to several hundreds of angstroms. While the buffer layer 11 isformed for the purpose of relaxing lattice constant mismatch between thesubstrate and the nitride semiconductor, it may be omitted depending onthe method of growing the nitride semiconductor, type of substrate andother conditions.

(Second Buffer Layer 112)

The second buffer layer 112 is a layer made of a single crystal nitridesemiconductor which is grown on the buffer layer 11 at a temperaturehigher than that of the buffer layer, and is formed to be thicker thanthe buffer layer 11. The second buffer layer 112 is made as a layerwhich has a concentration of n-type impurity lower than that of then-side contact layer 12 to be grown next, or as a nitride semiconductorlayer which is not doped with an n-type impurity. The second bufferlayer 112 may be constituted as In_(X)Al_(Y)Ga_(1-X-Y)N (0≦X, 0≦Y,X+Y≦1), for example, of which composition is not a matter of importancehere, but the composition is preferably in the form of Al_(Y)Ga_(1-Y)Nwhich is undoped and has a proportion of Al (value of Y) within 0.1, andmost preferably undoped GaN. When made in such a composition, the secondbuffer layer 112 having few defects can be grown and, when the secondbuffer layer 112 is made in the form of undoped GaN, the second bufferlayer 112 having fewest defects can be formed. Also it is furtherpreferable that the n-side contact layer 12 be formed in super latticestructure.

When the second buffer layer 112 having low impurity concentration andfew defects is grown before growing the n-side contact layer 12, asdescribed above, the n-side contact layer 12 having a high carrierconcentration and few defects can be formed with a relatively largerthickness. That is, while the n-side contact layer 12 having a highcarrier concentration needs to be formed by growing a nitridesemiconductor of a high n-type impurity concentration, it is difficultto grow a thick nitride semiconductor layer having a high impurityconcentration with few defect. Thus when the n-side contact layer 12 isformed without forming the n-side buffer layer 112, not only the n-sidecontact layer 12 having many defects is formed but also other nitridesemiconductor such as active layer comes to be grown on the n-sidecontact layer having many defects, causing the layer to be formedthereon to include crystal defect extending from the layer below, makingit impossible for the layer (active layer or the like) to be formedthereon to grow with few defects. Therefore, the second buffer layer 112has important roles in growing the n-side contact layer to be formedthereon with few defects as well as in forming the semiconductor layerswhich constitute the nitride semiconductor device with few defect.

Thickness of the second buffer layer 112 is preferably controlled to be0.1 μm or greater, more preferably 0.5 μm or greater, and mostpreferably in a range from 1 μm to 20 μm. When the second buffer layer112 is thinner than 0.1 μm, substantial improvement in the quality ofcrystal of the n-side contact layer 12 cannot be expected. When thesecond buffer layer 112 is thicker than 20 μm, on the other hand, thesecond buffer layer 112 itself tends to include much crystal defectsresulting in decreased effect of buffer layer. When the second bufferlayer 112 is grown to be relatively thick to an extent that does notexceed 20 μm, there is an advantage of improved heat dissipation. Thatis, when a laser device is made, heat is more easily transmitted in thesecond buffer layer 112 and therefore life of the laser device iselongated. Moreover, leaking light from the laser light diffuses in thesecond buffer layer 112, thereby making it easier to obtain laser beamhaving near elliptical configuration. The second buffer layer 112 may beomitted when an electrically conductive substrate such as GaN, SiC, ZnOor the like is used as the substrate.

(n-Side Contact Layer 12)

The n-side contact layer 12 acts as a contact layer whereon a negativeelectrode is formed, of which thickness is preferably controlled withina range from 0.2 μm to 4 μm. When the thickness is less than 0.2 μm, itbecomes difficult to control the etching rate for exposing the layer ina subsequent process of forming the negative electrode. When thethickness is greater than 4 μm, on the other hand, quality of crystaltend to become poorer due to impurity. The n-side contact layer 12 ismade of, for example, GaN doped with Si. Doping concentration of n-typeimpurity in the nitride semiconductor of the n-side contact layer 12 ispreferably in a range from 1×10¹⁷/cm³ to 1×10²¹/cm³, and more preferablyfrom 1×10¹⁸/cm³ to 1×10¹⁹/cm³. When the concentration is lower than1×10¹⁷/cm³, satisfactory ohmic contact with then electrode materialcannot be obtained and therefore threshold current and voltage cannot bedecreased in a laser device. When the concentration is higher than1×10²¹/cm³, leak current in the device increases and the quality ofcrystal deteriorate, resulting in a shorter device life. It is desirableto set the impurity concentration in the n-side contact layer 12 higherthan that of the n-cladding layer 14 thereby to increase the carrierconcentration in the n-side contact layer 12, in order to reduce theohmic contact resistance with the n electrode 23. When an electricallyconductive substrate such as GaN, SiC, ZnO or the like is used as thesubstrate and the negative electrode is installed on the back of thesubstrate, the n-side contact layer 12 acts as a buffer layer, not as acontact layer.

At least one of the second buffer layer 11 and the n-side contact layer12 may also be made in super lattice structure, in which case quality ofcrystal of the layer are drastically improved and the threshold currentcan be decreased. Preferably the n-side contact layer 12 which isthinner than the second buffer layer 11 is made in super latticestructure. When the n-side contact layer 12 is made in such a superlattice structure that the first layer and the second layer havingdifferent levels of band gap energy are laminated, contact resistancewith the n electrode 23 can be decreased and the value of threshold canbe decreased by exposing the layer having lower band gap energy therebyto form the n electrode 23. As materials to make the n electrode 23 forproviding favorable ohmic contact with the n-type nitride semiconductor,there are metals such as Al, Ti, W, Si, Zn, Sn and In, and alloysthereof.

When the n-side contact layer 12 is made in super lattice structure ofdifferent impurity concentration, resistance in the transverse directioncan be reduced due to an effect similar to that of HEMT described inconjunction with the first embodiment, thereby making it possible toreduce the threshold voltage and current of the LD device.

(Crack Preventing Layer 13)

The crack preventing layer 13 is made of, for example, In_(0.1)Ga_(0.9)Ndoped with Si in a concentration of 5×10¹¹/cm³ and has a thickness of,for example, 500 angstroms. The crack preventing layer 13, when formedby growing an n-type nitride semiconductor which includes In, preferablyInGaN, is capable of preventing cracks from developing in a nitridesemiconductor layer which includes Al to be formed thereon. The crackpreventing layer 13 is preferably grown to a thickness in a range from100 angstroms to 0.5 μm. When the thickness is less than 100 angstroms,the layer is difficult to function as a crack preventing layer. When thethickness is greater than 0.5 μm, the crystal tends to be blackened. Thecrack preventing layer 13 may be omitted in case the n-side contactlayer 12 is made in super lattice structure as in the case of the firstembodiment, or when the n-side cladding layer 14 to be grown next ismade in super lattice structure.

(n-Side Cladding Layer 14 Made in n-Type Super Lattice Structure)

The n-side cladding layer is made of n-type Al_(0.2)Ga_(0.8)N doped withSi in a concentration of 5×10¹⁸/cm³ in super lattice structure made bylaminating the first layer 20 angstroms thick and the second layer madeof undoped GaN 20 angstroms thick alternately, and has an overallthickness of, for example, 0.5 μm. The n-side cladding layer 14functions as a carrier trapping layer and light trapping layer and, whenit is made in super lattice structure, one of the layers is preferablymade by growing a nitride semiconductor which includes Al, preferablyAlGaN. When the layer is grown to a thickness not less than 100angstroms and within 2 μm, more preferably in a range from 500 angstromsto 1 μm, a good carrier trapping layer can be formed. While the n-sidecladding layer 14 may be made by growing a single nitride semiconductor,it may also be made in a super lattice layer which enables it to form acarrier trapping layer of good quality of crystal without cracks.

(n-Side Optical Waveguide Layer 15)

The n-side optical waveguide layer 15 is made of, for example, n-typeGaN doped with Si in a concentration of 5×10¹⁸/cm³ and has a thicknessof 0.1 μm. The n-side optical waveguide layer 15 functions as an opticalwaveguide layer for the active layer and is preferably formed by growingGaN or InGaN to a thickness usually in a range from 100 angstroms to 5μm, more preferably in a range from 200 angstroms to 1 μm. The opticalwaveguide layer 15 may also be made in super lattice structure. In casethe n-side optical waveguide layer 15 and the n-side cladding layer 14are made in super lattice structure, mean band gap energy of the nitridesemiconductor layers which constitute the super lattice layer is set tobe higher than that of the active layer. When forming a super latticelayer, at least one of the first layer and the second layer may or maynot be doped with an n-type impurity. The n-side optical waveguide layer15 may also be either a single layer of undoped nitride semiconductor ora super lattice layer made by laminating undoped nitride semiconductors.

(Active Layer 16)

The active layer 16 is made by alternately laminating a quantum welllayer which is made of, for example, In_(0.2)Ga_(0.8)N doped with Si ina concentration of 8×10¹⁸/cm³ and has a thickness of 25 angstroms and abarrier layer made of In_(0.05)Ga_(0.95)N doped with Si in aconcentration of 8×10¹⁸/cm³ and has a thickness of 50 angstroms, therebyforming a layer of multiple quantum well structure (MQW) having aspecified thickness. In the active-layer 16, either both or one of thequantum well layer and the barrier layer may be doped with the impurity.When doped with an n-type impurity, threshold value tends to decrease.When the active layer 16 is made in multiple quantum well structure asdescribed above, it is always formed by laminating the quantum welllayer having a lower band gap energy and a barrier layer having a bandgap energy lower than that of the quantum well layer, and is thereforedistinguished from super lattice layer. Thickness of the quantum welllayer is within 100 angstroms, preferably within 70 angstroms and mostpreferably 50 angstroms. Thickness of the barrier layer is within 150angstroms, preferably within 100 angstroms and most preferably 70angstroms. For example, the quantum well structure active layerdisclosed by Japanese Non-examined Patent Publication No. 9-148678 (U.S.patent application Ser. No. 08/743,72).

(p-Side Cap Layer 17)

The p-side cap layer 17 has band gap energy higher than that of theactive layer 16, and is made of, for example, p-type Al_(0.3)Ga_(0.7)Ndoped with Mg in a concentration of 1×10²⁰ cm³ and has a thickness of,for example, 200 angstroms. While the cap layer 17 is preferably used inthis way according to the second embodiment, the cap layer is formedwith a small thickness and therefore may be of i-type wherein carriersare compensated by doping n-type impurity. Thickness of the p-side caplayer 17 is controlled within 0.11═, more preferably within 500angstroms, and most preferably within 300 angstroms. When grown to athickness greater than 0.1 μm cracks tend to develop in the p-side caplayer 17 making it difficult to grow a nitride semiconductor layer ofgood quality of crystal. When the thickness of the p-side cap layer 17is greater than 0.1 μm, the carrier cannot pass through the p-side caplayer 17, which acts as an energy barrier, through tunnel effect. Whenthe penetration of carrier by tunnel effect is taken into consideration,the thickness is preferably set to within 500 angstroms and morepreferably within 300 angstroms.

The p-side cap layer 17 is preferably formed by using AlGaN having ahigh proportion of Al in order to make the LD device easier tooscillate, and the LD device becomes easier to oscillate when the AlGaNlayer is made thinner. For example, in the case of Al_(Y)Ga_(1-Y)N withthe value of Y being 0.2 or greater, the thickness is preferablycontrolled within 500 angstroms. While the lower limit of the p-side caplayer 17 is not specified, it is preferably formed to a thickness notless than 10 angstroms.

(p-Side Optical Waveguide Layer 18)

The p-side optical waveguide layer 18 has band gap energy higher thanthat of the p-side cap layer 17 and is made of, for example, p-type GaNdoped with Mg in a concentration of 1×10²⁰/cm³ and has a thickness of0.1 μm. The p-side optical waveguide layer 18 functions as an opticalwaveguide layer for the active layer 16, and is preferably formed bygrowing GaN, InGaN similarly to the n-side optical waveguide layer 15.This layer functions also as a buffer layer when growing the p-sidecladding layer 19, and functions as a favorable optical waveguide layerwhen grown to a thickness in a range from 100 angstroms to 5 μm, andmore preferably in a range from 200 angstroms to 1 μm. While the p-sideoptical waveguide layer is usually rendered p-type conductivity bydoping a p-type impurity such as Mg, alternatively it may not be dopedwith any impurity. And the p-side optical waveguide layer may also bemade in super lattice structure. When made in super lattice structure,at least one of the first layer and the second layer may be doped with ap-type impurity, or may not be doped at all.

(p-Side Cladding Layer 19=Super Lattice Layer)

The p-side cladding layer 19 is made of p-type Al_(0.2)Ga_(0.8)N dopedwith Mg in a concentration of 1×10²⁰/cm³ in super lattice structure madeby alternately laminating the first layer 20 angstroms thick and thesecond layer which is made of p-type GaN and is doped with Mg in aconcentration of 1×10²⁰/cm³ having a thickness of 20 angstroms. Thep-side cladding layer 19 functions as a carrier trapping layer similarlyto the n-side cladding layer 14 and particularly functions as a layerfor decreasing the resistivity of the p-type layer. While limits ofthickness of the p-side cladding layer 19 are not specified, too, it ispreferably formed to a thickness not less than 100 angstroms and notgreater than 2 _m, more preferably not less than 500 angstroms and notgreater than 1 μm.

(p-Side Contact Layer 20)

The p-side contact layer 20 is made of, for example, p-type GaN dopedwith Mg in a concentration of 2×10²⁰/cm³ and has a thickness of, forexample, 150 angstroms. The p-side contact layer 20 can be made in aconstitution of In_(X)Al_(Y)Ga_(1-X-Y)N (0≦X, 0≦Y, X+Y≦1) of p-type, andthe most preferable ohmic contact with the p electrode 21 can beobtained by using GaN doped with Mg as described above. Thickness of thep-type contact layer is preferably controlled to within 500 angstroms,more preferably within 300 angstroms and most preferably within 200angstroms. This is because resistivity can be further decreased bycontrolling the thickness of the p-type nitride semiconductor which hasa high resistivity of several ohm-centimeters or higher within 500angstroms, thus reducing the threshold current and voltage. It is alsomade possible to increase the amount of hydrogen removed from the p-typelayer, thereby further reducing the resistivity.

According to the present invention, the p-side contact layer 20 may alsobe made in super lattice structure. When making a super lattice layer,the first layer and the second layer having different values of band gapenergy are laminated in a succession of 1st, 2nd, 1st, 2nd and so on, sothat the layer having lower band gap energy is exposed at the last, inwhich case preferable ohmic contact with the p electrode 21 can beobtained. The p electrode 21 may be made of, for example, Ni, Pd, Ni/Au,etc.

According to the second embodiment, an insulating film 25 made of SiO₂is formed on the surface of nitride semiconductor layer exposed betweenthe p electrode 21 and an n electrode 23 as shown in FIG. 2, while the ppad electrode 22 which is electrically connected to the p electrode 21via an aperture formed in the insulating film 25 and an n pad electrode24 connected to the n electrode 23 are formed. The p pad electrode 22increases the substantial surface of the p pad electrode 22 so that theside of p electrode can be wire-bonded and die-bonded, and the n padelectrode 24 prevents the n electrode 23 from peeling off.

The nitride semiconductor device of the second embodiment has the p-typecladding layer 19 of good quality of crystal which is a super latticelayer made by laminating the first layer and the second layer with afilm thickness within the elastic strain limit. With this configurationof the nitride semiconductor device according to the second embodiment,resistance of the p-side cladding layer 19 can be decreased to a levelat least one order of magnitude lower than that of a p-type claddinglayer which does not have super lattice structure, and thereforethreshold voltage and current can be reduced.

Also in the nitride semiconductor device of the second embodiment, anitride semiconductor having a low band gap energy is formed as thep-side contact layer 20 having a small thickness of 500 angstroms orless in contact with the p-side cladding layer 19 which includes p-typeAl_(Y)Ga_(1-Y)N, so that the carrier concentration of the p-side contactlayer 20 is substantially increased and favorable ohmic contact with thep electrode can be obtained, thereby making it possible to decrease thethreshold current and voltage of the device. Further, because the secondbuffer layer 112 is installed before growing the n contact layer, thenitride semiconductor layer to be grown on the second buffer layer 112can be rendered good quality of crystal and a long-life device can bemade. When the n-side contact layer which is grown on the second bufferlayer 112 is made in super lattice structure, resistance in thetransverse direction is reduced thereby making it possible to make adevice having low threshold voltage and threshold current.

In case a nitride semiconductor which includes at least indium, such asInGaN, is provided in the active layer 16 in the LD device of the secondembodiment 2, it is preferable that a super lattice layer made byalternately laminating In_(X)Ga_(1-X)N and Al_(Y)Ga_(1-Y)N is used aslayers which interpose the active layer 16 (the n-side cladding layer 14and the p-side cladding layer 19). When configured in this way,differences in the band gap energy and refractive index between theactive layer 16 and the super lattice layer can be increased, so thatthe super lattice layer can act as an excellent light trapping layerwhen a laser device is manufactured. Moreover, because InGaN has softercrystal characteristic than other nitride semiconductors which includeAl such as AlGaN, cracks are less likely to develop throughout thelaminated nitride semiconductor layers when InGaN is used in the activelayer. Thus the service life of the LD device can be elongated.

In the case of a semiconductor device of double-hetero structure havingthe active layer 16 of quantum well structure as in the secondembodiment, it is preferable that the p-side cap layer 17 made of anitride semiconductor which has band gap energy higher than that of theactive layer 16 and a thickness within 0.1 μm, preferably the p-side caplayer 17 made of a nitride semiconductor which includes Al is installedin contact with the active layer 16, the p-side optical waveguide layer18 having a band gap energy lower than that of the p-side cap layer 17is installed at a position farther than the p-side cap layer 17 from theactive layer, and a nitride semiconductor having a band gap energyhigher than that of the p-side optical waveguide layer 18, preferablythe p-side cladding layer 19 having super lattice structure whichincludes a nitride semiconductor including Al, is installed at aposition farther than the p-side optical waveguide layer 18 from theactive layer. Because the p-side cap layer 17 is made with a high bandgap energy, electrons injected from the n layer are blocked by thep-side cap layer 17 and trapped, so that the electrons do not overflowthe active layer, thus resulting in less leakage current in the device.

While configurations of the nitride semiconductor device according tothe second embodiment which are preferable as the configuration of laserdevice are described above, the present invention is not limited to anyparticular device configuration and is applicable as long as at leastone n-type super lattice layer is provided in the semiconductor region202 on n conductivity side below the active layer 16, and at least onep-type super lattice layer is provided in the semiconductor region 252on p conductivity side above the active layer 16. However, it is mostpreferable for the purpose of reducing the values of Vf and threshold ofthe device, that the p-type super lattice layer is formed in the p-sidecladding layer 19, which acts as a carrier trapping layer, when providedin the semiconductor region 252 on p conductivity side, and is formed inthe n contact layer 12, which acts as a current injection layer whichthe n electrode 23 is in contact with, or the n cladding layer 14, whichacts as a carrier trapping layer, when provided in the semiconductorregion 202 on n conductivity side. It is a matter of course that theconfiguration similar to that of the device of the second embodiment canbe applied to an LED device (except for that a ridge is not necessaryfor the LED device).

In the nitride semiconductor device of the second embodiment configuredas described above, it is preferable that the device be annealed in anatmosphere which does not include H, for example nitrogen atmosphere, ata temperature not lower than 400° C., for example 700° C., after thelayers have been formed. This process decreases the resistance of eachlayer of the p-type nitride semiconductor region further, therebydecreasing the threshold voltage further.

Also in the nitride semiconductor device of the second embodiment, the pelectrode 21 made of Ni and Au is formed in the form of stripe on thesurface of the p-side contact layer 12, while the n-side contact layeris exposed symmetrically with respect to the p electrode 21 and the nelectrode 23 is installed to cover nearly the entire surface of then-side contact layer. When an insulating substrate is used, such aconfiguration of providing the n electrode 23 symmetrically on bothsides of the p electrode 21 is very advantageous in decreasing thethreshold voltage.

According to the second embodiment, a multi-layer dielectric film madeof SiO₂ and TiO₂ may be formed on the cleavage plane (plane ofresonator) which is perpendicular to the ridge (stripe-shapedelectrode).

According to the present invention, because the super lattice layer isused either as the cladding layer which acts as the carrier trappinglayer formed in the n-type region or p-type region interposing theactive layer, or as an optical waveguide layer of the active layer or asa current injection layer provided with an electrode being formed incontact therewith, it is preferable to control the mean band gap energyof the nitride semiconductor which constitutes the super lattice layerto be higher than that of the active layer.

Embodiment 3

FIG. 3 is a schematic cross sectional view showing the configuration ofthe nitride semiconductor device according to the third embodiment ofthe present invention. The nitride semiconductor device of the thirdembodiment is a laser diode of stripe electrode type using the end faceof the active layer as a resonance plane, and FIG. 3 schematically showsthe cross section of the device when cut in a direction perpendicular tothe direction of propagation of laser light. The third embodiment of thepresent invention will be described below by referring to FIG. 3.

In FIG. 3, reference numerals refer to the following components.

100 denotes a GaN substrate of a thickness not less than 10 μm which isgrown on an auxiliary substrate made of a material other than nitridesemiconductor, for example a substrate made of such material assapphire, spinel, SiC, Si, GaAs and ZnO. The auxiliary substrate may beremoved after the GaN substrate 100 is formed as shown in FIG. 3, or maybe left to remain thereon and used without removing as in the case of anembodiment to be described later (FIG. 8).

11 denotes a buffer layer made of Si-doped n-type GaN, which alsofunctions as an n-side contact layer in the third embodiment.

14 denotes an n-side cladding layer of super lattice structure made bylaminating Si-doped n-type Al_(0.2)Ga_(0.8)N (first nitridesemiconductor layer) 40 angstroms thick and undoped GaN layer (secondnitride semiconductor layer) 40 angstroms thick, alternately in 100layers. In the third embodiment, the n-side cladding layer 14 is formedat a position located apart from the active layer.

15 denotes an n-side guide layer made of, for example, undoped GaN whichis placed between the n-side cladding layer 14 and the active layer 16and has a band gap energy lower than that of Al_(0.2)Ga_(0.8)N of then-side cladding layer 14.

A semiconductor region 203 of n conductivity side is constituted fromthe n-side buffer layer 11, the n-side cladding layer 14 and the n-sideoptical waveguide layer 15.

16 denotes an active layer of multi-quantum well structure made bylaminating three quantum well layers made of Al_(0.2)Ga_(0.8)N with athickness of 30 angstroms and two barrier layers made ofIn_(0.05)Ga_(0.95)N having a band gap energy higher than that of thequantum well layers and thickness of 30 angstroms, alternately, in fivelayers in all.

17 denotes a p-side cap layer made of, for example, Mg-doped p-typeAl_(0.3)Ga_(0.7)N which has a band gap energy higher than the band gapenergy of the quantum well layers of the active layer 16 and higher thanthe band gap energy of the p-side optical waveguide layer 18. The bandgap energy of the p-side cap layer 17 is preferably set to be higherthan that of the nitride semiconductor layer (fourth nitridesemiconductor layer) which has less band gap energy among the p-sidecladding layer 19 of the super lattice structure.

18 denotes the p-side guide layer made of, for example, undoped GaNwhich is placed between the p-side cladding layer 19 and the activelayer 16 and has a band gap energy lower than that of Al_(0.2)Ga_(0.8)Nof the p-side cladding layer 19.

19 denotes a p-side cladding layer of super lattice structure made bylaminating Mg-doped p-type Al_(0.2)Ga_(0.8)N which is 40 angstroms thickand undoped GaN layer 40 angstroms thick, alternately in 100 layers,located apart from the active layer.

20 denotes a p-side contact layer made of, for example, Mg-doped GaNwhich has a band gap energy lower than that of the Al_(0.2)Ga_(0.8)N ofthe p-side cladding layer 19.

A semiconductor region 253 of p conductivity side is constituted fromthe p-side cap layer 17, the p-side optical waveguide layer 18, thep-side cladding layer 19 and the p-side contact layer 20.

As described above, the laser device of the third embodiment of thepresent invention has such a structure as the nitride semiconductorlayers 11 and 14 through 20 are laminated on the GaN substrate 100,wherein stripe ridge is formed on the nitride semiconductor layerlocated over the p-side cladding layer 19 and the p electrode 21 isformed on substantially the entire surface of the p-side contact layer20 located on the outermost surface of the ridge. On the surface (topsurface) of the exposed n-side buffer layer 11, on the other hand, isformed the n electrode 23. In the third embodiment, the n electrode 23is formed on the surface of the n-side buffer layer 11, but because theGaN substrate 100 is used as the substrate, such a configuration mayalso be employed as the portion where the n electrode is to be formed isetched down to the GaN substrate 100 thereby to expose the surface ofthe GaN substrate 100, then the n electrode is formed on the exposed GaNsubstrate 100 so that the p electrode and the n electrode are providedon the same side. An insulating film 25 made of, for example, SiO₂ isformed on the exposed surface of the nitride semiconductor except forthe top of the n electrode 23 and the p electrode 21, while the n padelectrode 22 and the n pad electrode 24 are provided for the purpose ofbonding for the connection with the p electrode 21 and the n electrode23 via the apertures in the insulating film 25 above the n electrode 23and the p electrode 21. While the region comprising the nitridesemiconductor layers placed between the active layer and the p electrodeis referred to as the semiconductor region of p conductivity side, thisdoes not mean that all the nitride semiconductor layers constituting thesemiconductor region have p-type conductivity. Similarly, the regioncomprising the nitride semiconductor layers placed between the activelayer and a GaN substrate 100 is referred to as the semiconductor regionof n-side conductivity, this does not mean that all the nitridesemiconductor layers constituting the region have n-type conductivity,as described previously.

The laser device according to the third embodiment of the presentinvention has the n-side cladding layer 14 of super lattice structuremade by laminating the first nitride semiconductor layer having a highband gap energy and the second nitride semiconductor layer having a bandgap energy lower than that of the first nitride semiconductor layer, thetwo layers having different impurity concentrations, provided atpositions apart from the active layer 16 in the n-side nitridesemiconductor layer located below the active layer 16 shown in FIG. 3.Thickness of the first nitride semiconductor layer and the secondnitride semiconductor layer which constitute the super lattice layer ispreferably controlled to be within 100 angstroms, more preferably within70 angstroms and most preferably within a range from 10 to 40 angstroms.When the thickness is greater than 100 angstroms, the first nitridesemiconductor layer and the second nitride semiconductor layer becomethicker than the elastic strain limit and microscopic cracks or crystaldefect tend to develop in the film. While the lower limit of thethickness of the first nitride semiconductor layer and the secondnitride semiconductor layer is not specified according to the presentinvention and may be of any value as long as it is monoatomic layer orthicker, it is preferably 10 angstroms or greater. Further, the firstnitride semiconductor layer is preferably made by growing a nitridesemiconductor which includes at least Al, preferably Al_(X)Ga_(1-X)N(0<X≦1). While the second nitride semiconductor layer may be anything aslong as it is a nitride semiconductor having a band gap energy lowerthan that of the first nitride semiconductor layer, it is preferablymade of a nitride semiconductor of binary mixed crystal or ternary mixedcrystal such as Al_(Y)Ga_(1-Y)N (0≦Y≦1, X>Y) and In_(Z)Ga_(1-Z)N (0≦Z<1)which can be grown easily and provide good quality of crystal. Accordingto the present invention, it is more preferable that the first nitridesemiconductor be Al_(X)Ga_(1-X)N (0<X<1) which does not include In andthe second nitride semiconductor be In_(Z)Ga_(1-Z)N (0≦Z<1) which doesnot include Al, and most preferably the first nitride semiconductor isAl_(X)Ga_(1-X)N (0<X≦0.3) with the mixing proportion of Al (value of Y)being 0.3 or less and the second nitride semiconductor is GaN, for thepurpose of obtaining super lattice of excellent quality of crystal.

When the first nitride semiconductor is made by using Al_(X)Ga_(1-X)N(0<X<1) and the second nitride semiconductor is made by using GaN, aremarkable advantage in terms of manufacture as described below can beobtained. That is, when forming the Al_(X)Ga_(1-X)N (0<X<1) layer andthe GaN layer by metal organic vapor phase deposition process (MOVPE:metal organic vapor phase epitaxy), every layer can be grown in the sameH₂ atmosphere. Therefore, a super lattice layer can be formed by growingthe Al_(X)Ga_(1-X)N (0<x<1) layer and the GaN layer alternately withoutchanging the atmosphere. This provides a remarkable advantage whenmanufacturing the super lattice layer which requires several tens toseveral hundreds of layers to be laminated.

The cladding layer which has the functions of light trapping layer andcarrier trapping layer must have a band gap energy higher than that ofquantum well structure of the active layer. Although a nitridesemiconductor of high mixing proportion of Al can be used to make anitride semiconductor layer of a relatively high band gap energy, it hasbeen very difficult to grow a crystal of nitride semiconductor of highmixing proportion of Al, because of cracks which are likely to developin a thick film. When formed in a super lattice layer as in the case ofthe present invention, however, cracks are made less likely to occurbecause the crystal is grown to a thickness within the elastic strainlimit, even when the AlGaN layer formed as the first nitridesemiconductor layer constituting the super lattice layer is made with asomewhat high mixing proportion of Al. With this configuration, in thepresent invention, a layer having a high mixing proportion of Al can begrown with good quality of crystal, and therefore it is made possible toform a cladding layer having good effects of light trapping and carriertrapping, thereby reducing the threshold voltage in the laser device.The present invention can also be applied to LED devices, in which caseit is made possible to decrease Vf (forward voltage) in the LED device.

Further in the laser device according to the third embodiment of thepresent invention, n-type impurity concentration is set to be differentbetween the first nitride semiconductor layer and the second nitridesemiconductor layer of the n-side cladding layer 14. This configurationis the so-called modulation doping. When one layer is made with lowern-type impurity concentration or is preferably undoped with the impurityand the other layer is doped in a higher concentration, this modulationdoping is also capable of decreasing the threshold voltage and Vf. Thisis because the presence of a layer having a low impurity concentrationin the super lattice layer increases the mobility in the layer, andcoexistence of a layer having a high concentration of impurity makes itpossible to form a super lattice layer even when the carrierconcentration is high. That is, it is supposed that the coexistence of alayer of low impurity concentration and high mobility and a layer ofhigh impurity concentration and high carrier concentration allows alayer having a high impurity concentration and high mobility to be acladding layer, thus decreasing the threshold voltage and vf.

When a nitride semiconductor layer having a high band gap energy isdoped with an impurity in a high concentration, the modulation dopingeffect is supposed to generate two dimensional electron gas between ahigh impurity concentration layer and a low impurity concentrationlayer, so that the resistivity decreases due to the effect of the twodimensional electron gas. In a super lattice layer made by laminating anitride semiconductor layer which is doped with an n-type impurity andhas a high band gap energy and an undoped nitride semiconductor layerwith a low band gap energy, for example, the barrier layer side isdepleted in the hetero-junction interface between the layer which isdoped with the n impurity and the undoped layer, while electrons (twodimensional electron gas) accumulate in the vicinity of the interface onthe side of the layer having lower band gap. Since the two dimensionalelectron gas is formed on the lower band gap side and therefore theelectron movement is not subject to disturbance by the impurity,electron mobility in the super lattice increases and the resistivitydecreases. It is also supposed that the modulation doping on p side iscaused by the effect of the two dimensional positive hole gas. In thecase of p layer, AlGaN has higher resistivity than GaN has. Thus it issupposed that, because the resistivity is decreased by doping AlGaN withp type impurity in a higher concentration, a substantial decrease iscaused in the resistivity of the super lattice layer, thereby making itpossible to decrease the threshold value when the laser device is made.

When a nitride semiconductor layer having a low band gap energy is dopedwith an impurity in a high concentration, such an effect as describedbelow is expected to be produced. When the AlGaN layer and the GaN layerare doped with the same amounts of Mg, for example, acceptor level of Mgbecomes deeper and the activation ratio becomes lower in the AlGaNlayer. In the GaN layer, on the other hand, acceptor level of Mg becomesless deep and the Mg activation ratio becomes higher than in the AlGaNlayer. When doped with Mg in a concentration of 1×10²⁰/cm³, for example,carrier concentration of about 1×10¹⁸/cm³ is obtained in GaN, while theconcentration obtained in AlGaN is only about 1×10¹⁷/cm³. Hence in thepresent invention, a super lattice layer is made from AlGaN and GaN andthe GaN layer from which higher carrier concentration can be expected isdoped with greater amount of impurity, thereby forming super lattice ofa high carrier concentration. Moreover, because tunnel effect causes thecarrier to move through the AlGaN layer of a lower impurityconcentration due to the super lattice structure, the carrier can movein the AlGaN layer under almost no influence of the impurity, while theAlGaN layer functions also as a cladding layer having a high band gapenergy. Therefore, even when the nitride semiconductor layer of lowerband gap energy is doped with a greater amount of impurity, very goodeffect can be obtained in decreasing the threshold voltage of the laserdevice or LED device. The above description deals with a case of formingthe super lattice layer on p-type layer side, although similar effectcan be obtained also when a super lattice layer is formed on the n layerside.

When the first nitride semiconductor layer having a higher band gapenergy is doped with an n-type impurity in a high concentration, theamount of doping in the first nitride semiconductor layer is preferablycontrolled within a range from 1×10¹⁷/cm³ to 1×10²⁰/cm³, or morepreferably within a range from 1×10¹⁸/cm³ to 5×10¹⁹/cm³. When theimpurity concentration is lower than 1×10¹⁷/cm³, the difference from theconcentration in the second nitride semiconductor layer becomes toosmall to obtain a layer of high carrier concentration. When the impurityconcentration is higher than 1×10²⁰/cm³, on the other hand, leak currentin the device itself tends to increase. Meanwhile the n-type impurityconcentration in the second nitride semiconductor layer may be at anylevel as long as it is lower than that of the first nitridesemiconductor layer, but it is preferably lower than one tenth of thelatter. Most preferably the second nitride semiconductor layer isundoped, in which case a layer of the highest mobility can be obtained.However, because each of the component layers of a super lattice layeris thin, some of the n-type impurity diffuses from the first nitridesemiconductor into the second semiconductor layer. Though even in thiscase, effects of the present invention can be obtained when the n-typeimpurity concentration in the second nitride semiconductor layer iswithin 1×10¹⁹/cm³. The n-type impurity is selected from among theelements of VB group and VIB group of the periodic table such as Si, Ge,Se, S and O, and preferably selected from among Si, Ge and S. The effectis the same also in case the first nitride semiconductor layer having ahigher band gap energy is doped with less amount of n-type impurity andthe second nitride semiconductor layer having a lower band gap energy isdoped with greater amount of N-type impurity.

The laser device according to the third embodiment of the presentinvention has the p-side cladding layer 19 of super lattice structuremade by laminating a third nitride semiconductor layer having a highband gap energy and a fourth nitride semiconductor layer having a bandgap energy lower than that of the third nitride semiconductor layer, thetwo layers having different impurity concentrations, provided atpositions apart from the active layer 16 in the p-side nitridesemiconductor layer located above the active layer 16 shown in FIG. 3.Thickness of the third nitride semiconductor layer and the fourthnitride semiconductor layer which constitute the super lattice layer arepreferably controlled to be within 100 angstroms, more preferably within70 angstroms and most preferably within a range from 10 to 40 angstroms,as in the case of the n-side cladding layer 14. Similarly, the thirdnitride semiconductor layer is preferably made by growing a nitridesemiconductor which includes at least Al, preferably Al_(X)Ga_(1-X)N(0<X≦1). The fourth nitride semiconductor is preferably made of anitride semiconductor of binary mixed crystal or ternary mixed crystalsuch as Al_(Y)Ga_(1-Y)N (0≦Y≦1, X>Y) or In_(Z)Ga_(1-Z)N (0≦z≦1).

When the p-type cladding layer 19 is made in super lattice structure,the super lattice structure has the same effect on the laser device asthat of the n cladding layer 14, and also has such an effect as followsin addition to the case of forming on the n layer side. That is, thep-type nitride semiconductor has resistivity which is usually at leasttwo orders of magnitude higher compared to n-type nitride semiconductor.Therefore, when a super lattice layer is formed on the p layer side,remarkable effect of reducing the threshold voltage is obtained. Nitridesemiconductor is known to be a semiconductor which is difficult toobtain in the form of p-type crystal. Such a process has been knownwhere a nitride semiconductor layer doped with a p-type impurity isannealed and then hydrogen is removed thereby manufacturing a p-typecrystal (Japanese Patent No. 2540791). However, the p-type nitridesemiconductor thus obtained has resistivity as high as severalohm-centimeters at the lowest. By turning this p-type layer into a superlattice layer, better quality of crystal can be obtained and theresistivity can be reduced at least one order of magnitude lower thanthat of the prior art, thereby making it possible to decrease thethreshold voltage.

In the third embodiment, the third nitride semiconductor layer and thefourth nitride semiconductor layer of the p-type cladding layer 19 areset to have different concentrations of p-type impurity, so that onelayer has a high impurity concentration and the other layer has a lowerimpurity concentration. Similarly to the case of the n-side claddinglayer 14, threshold voltage and Vf can be decreased by doping the thirdnitride semiconductor layer which has a high band gap energy with thep-type impurity in a higher concentration and doping the fourth nitridesemiconductor layer which has a low band gap energy with the p-typeimpurity in a lower concentration.

A configuration reverse to the above is also possible: the third nitridesemiconductor layer which has high band gap energy is doped with thep-type impurity in a lower concentration and the fourth nitridesemiconductor layer which has low band gap energy is doped with thep-type impurity in a higher concentration. The reason is as describedpreviously.

The amount of doping in the third nitride semiconductor layer ispreferably controlled within a range from 1×10¹⁸/cm³ to 1×10²¹/cm³, ormore preferably within a range from 1×10¹⁸/cm³ to 5×10²⁰/cm³. When theimpurity concentration is lower than 1×10¹⁸/cm³, the difference from theconcentration in the fourth nitride semiconductor layer becomes toosmall to obtain a layer of high carrier concentration. When the impurityconcentration is higher than 1×10²¹/cm³, on the other hand, quality ofcrystal tend to deteriorate. Meanwhile the p-type impurity concentrationin the fourth nitride semiconductor layer may be at any level as long asit is lower than that of the third nitride semiconductor layer, but itis preferably lower than one tenth of the latter. Most preferably thefourth nitride semiconductor layer is undoped, in order to obtain thehighest mobility. In practice, however, because the film is thin, someof the p-type impurity diffuses from the third nitride semiconductor. Inorder to obtain good effect in the present invention, the concentrationis preferably within 1×10²⁰/cm³. The p-type impurity is selected fromamong the elements of IIA group and IIB group of the periodic table suchas Mg, Zn, Ca and Be, and preferably selected from among Mg and Ca. Theabove applies also to such a case as the third nitride semiconductorlayer having higher band gap energy is doped with less amount of p-typeimpurity and the fourth nitride semiconductor layer having lower bandgap energy is doped with greater amount of p-type impurity.

In the nitride semiconductor layer constituting the super lattice layer,the layer doped with the impurity in a higher concentration ispreferably doped so that such a distribution of impurity concentrationis obtained, that the impurity concentration is high in the middleportion of the semiconductor layer in the direction of thickness(located far from the second nitride semiconductor layer or the fourthnitride semiconductor layer) and is low (or undoped) in the portionsnear the ends (portions adjacent to the second nitride semiconductorlayer and the fourth nitride semiconductor layer). When the superlattice layer is formed from the AlGaN layer doped with Si as n-typeimpurity and the undoped GaN layer, the AlGaN layer releases electronsas donor into the conductive band because it is doped with Si, theelectrons fall in the conductive band of the GaN which has a lowpotential. Because the GaN crystal is not doped with the donor impurity,carrier disturbance due to an impurity does not occur. Thus theelectrons can move easily in the GaN crystal, namely high electronmobility is obtained. This is similar to the effect of the twodimensional electron gas described previously, thus increasing themobility of the electrons substantially in the transverse direction anddecreasing the resistivity. In the AlGaN layer having a high band gapenergy, the effect is further enhanced when the central regionrelatively apart from the GaN layer is doped with the n-type impurity ina high concentration. That is, among the electrons that move in GaN,electrons passing through a portion near the AlGaN are more or lesssubject to disturbance by the n-type impurity ions (Si in this case)which are present in a portion of the AlGaN layer adjacent to the GaNlayer. However, when a portion of the AlGaN layer adjacent to the GaNlayer is undoped as described above, electrons passing through theportion near the AlGaN layer become less subject to the disturbance ofSi, and therefore mobility in the undoped GaN layer is further improved.Similar effect is obtained also when super lattice is formed from thethird nitride semiconductor layer of the p layer side and the fourthnitride semiconductor layer, although the action is different somewhat,and it is preferable that the third nitride semiconductor layer having ahigh band gap energy be doped with the p-type impurity in a higherconcentration at the middle portion thereof and doped in a lowerconcentration or undoped at all at portions which are adjacent to thefourth nitride semiconductor layer. Although the impurity concentrationdistribution may also be realized in the nitride semiconductor layerhaving a low band gap energy doped with the n-type impurity in a higherconcentration, a super lattice layer made by doping the nitridesemiconductor layer of a lower band gap energy has less effect.

While the configurations where the n-side cladding layer 14 and thep-side cladding layer 19 are made in super lattice layer structure havebeen described above, according to the present invention, the n-sidebuffer layer 11 acting as a contact layer, the n-side optical waveguidelayer 15, p-side cap layer 17, the p-side optical waveguide layer 18,p-side contact layer 20, or the like may also be formed in super latticestructure. That is, any layer, whether it makes contact with theactivation or not, may be formed in super lattice structure. When then-side buffer layer 11 whereon the n electrode is formed is formed insuper lattice structure, in particular, the effect similar to that ofHEMT can be easily obtained.

Further in the laser device according to the third embodiment of thepresent invention, the n-side optical waveguide layer 15 in which theimpurity (n-type impurity in this case) concentration is controlledwithin 1×10¹⁹/cm³ is formed between the n-side cladding layer 14 made ofsuper lattice layer and the active layer 16, as shown in FIG. 3. Evenwhen the n-side optical waveguide layer 15 is undoped, there is apossibility that the n-type impurity from other layers diffuses intothis layer, although the effect of the present invention will not belost provided that the concentration of the doped impurity is within1×10¹⁹/cm³, in which case the n-side optical waveguide layer 15functions as an optical waveguide layer. However, according to thepresent, invention, impurity concentration in the n-side opticalwaveguide layer 15 is preferably within 1×10¹⁸/cm³ more preferablywithin 1×10¹⁷/cm³, and most preferably the n-side optical waveguidelayer 15 is undoped. It is also preferable that the n-side opticalwaveguide layer 15 be formed from a nitride semiconductor which includesIn or from GaN.

Also in the laser device according to the third embodiment, the p-sideoptical waveguide layer 18 in which the impurity (p-type impurity inthis case) concentration is controlled within 1×10¹⁹/cm³ is formedbetween the p-side cladding layer 19 made of super lattice layer and theactive layer 16. According to the present invention, impurityconcentration in the p-side optical waveguide layer 18 may be within1×10¹⁹/cm³, but is preferably controlled within 1×10¹⁸/cm³ and mostpreferably the p-side optical waveguide layer 18 is undoped. While anitride semiconductor shows n-type conductivity when undoped,conductivity of the p-side optical waveguide layer 18 may be eithern-type or p-type according to the present invention and is referred toas p-side optical waveguide layer regardless of the conductivity type inthis specification. In practice, there is a possibility that the p-typeimpurity from other layers diffuses into thep-side optical waveguidelayer 18. It is preferable that the p-side optical waveguide layer bealso formed from a nitride semiconductor which includes In or from GaN.

The reason why it is preferable that an undoped nitride semiconductor beinterposed between the active layer and the cladding layer is asfollows. A nitride semiconductor is designed to have the active layeremit light of wavelength usually in a range from 360 to 520 nm, moreparticularly from 380 to 450 nm. An undoped nitride semiconductor haslower absorptance for light of the wavelength region described above,than nitride semiconductors doped with n-type impurity or p-typeimpurity. Therefore, when an undoped nitride semiconductor is interposedbetween a light emitting active layer and a cladding layer which acts asa light trapping layer, light emitted by the active layer is subject toless attenuation and it is made possible to make a laser device whichoscillates with a low gain while decreasing the threshold voltage. Thiseffect can be achieved when the impurity concentration in the opticalwaveguide layer is within 1×10¹⁹/cm³.

Thus a preferred embodiment of the present invention is a light emittingdevice which is located apart from an active layer and has a claddinglayer of super lattice structure that has been modulation-doped with animpurity, and a guide layer which is doped with an impurity in a lowconcentration or preferably undoped, interposed between the claddinglayer and the active layer.

Further preferred embodiment is the light emitting device of the thirdembodiment wherein a quantum well layer of active layer and the p-sidecap layer 17 made of a nitride semiconductor of thickness 0.1 μm or lesshaving a band gap energy higher than the band gap energy of theinterface of the p-side guide layer 18 is formed between the p-sideguide layer 18 and the active layer 16, with the impurity concentrationin the p-side cap layer being controlled at 1×10¹⁸/cm³ or higher.Thickness of the p-type cap layer 17 is controlled within 0.1 μm, morepreferably within 500 angstroms, and most preferably within 300angstroms. When grown to be thicker than 0.1 μm, cracks tend to developin the p-type cap layer 17 thus making it difficult to grow a nitridesemiconductor layer of good quality of crystal. When a thin layer havinga high band gap energy is formed to a thickness within 0.1 μm adjacentto the active layer, leak current in the light emitting device can bedecreased. This causes the electrons injected from the n layer side toaccumulate in the active layer due to the energy barrier of the caplayer, resulting in increased probability of recombination of electronsand positive holes, thereby making it possible to increase the outputpower of the device. It is necessary to control the impurityconcentration in the cap layer at 1×10¹⁸/cm³ or higher. That is, the caplayer has a relatively high proportion of mixed Al, and a layer having ahigh proportion of mixed Al tends to have high resistance. Therefore,unless the layer is doped with an impurity to increase the carrierconcentration and reduce the resistivity, the layer becomes similar to ilayer of high resistance, thus forming p-i-n structure with poorcurrent-voltage characteristics. The cap layer provided on the p sidemay also be formed on the n side. When formed on the n side, doping ofthe n-type impurity is not necessary.

In the laser device of the third embodiment constituted as describedabove, because the n-side cladding layer 14 and the p-side claddinglayer 19 are made in super lattice structure, electric resistance andthe threshold voltage of the n-side cladding layer 14 and the p-sidecladding layer 19 can be reduced and the laser can be oscillated for along period of time.

In the laser device of the third embodiment, reduction of the thresholdvoltage is attempted with various means as described above, in additionto the constitution of the n-side cladding layer 14 and the p-sidecladding layer 19 in super lattice structure.

Although the n-side cladding layer 14 and the p-side cladding layer 19are made in super lattice structure in the third embodiment, the presentinvention is not limited to this configuration and either one of then-side cladding layer 14 and the p-side cladding layer 19 may be made insuper lattice structure. With such a configuration, too, the thresholdvoltage can be reduced compared to the prior art.

Although the n-side cladding layer 14 and the p-side cladding layer 19are made in super lattice structure in the third embodiment, the presentinvention is not limited to this configuration and it suffices that anyone or more of the p-side and n-side nitride semiconductor layers otherthan the n-side cladding layer 14 and the p-side cladding layer 19 maybe made in super lattice structure. With such a configuration, too, thethreshold voltage can be reduced compared to the prior art.

Although the n-side cladding layer 14 and the p-side cladding layer 19are made in super lattice structure in the laser device of the thirdembodiment, the present invention is not limited to this configurationand can be applied to other nitride semiconductor devices such as lightemitting diode (LED), as a matter of course. With such a configuration,Vf (forward voltage) can be reduced in the case of a light emittingdiode.

As described above, because the laser device of the third embodiment hasthe cladding layer made in super lattice layer made by modulation dopingof an impurity, the threshold voltage can be decreased and oscillationcan be continued for a long period of time. Also the laser device can beset to a high characteristic temperature which makes it possible to makea good laser device. The characteristic temperature is the thresholdcurrent density as a function of temperature, which is proportional toexp (T/T₀), where T is the operating temperature (K) and T₀ is thecharacteristic temperature (K). In the laser device, higher value of T₀allows lower threshold current density even at a high temperature,resulting in stable operation. In the laser device of embodiment 27 tobe described later, for example, T₀ is as high as 150K or over. Thisvalue indicates excellent temperature characteristic of the LD. Thuswhen the laser device of the present invention is used as a writinglight source and/or reading light source, a capacity not obtained withthe prior art can be achieved, providing a great value of industrialutilization

Embodiment 4

FIG. 9 is a schematic perspective view showing the shape of the laserdevice according to the fourth embodiment of the present invention. FIG.9 also shows the cross section of the device when cut in a directionperpendicular to the ridge stripe. The fourth embodiment of the presentinvention will be described below by referring to FIG. 9.

Each layer of the laser device according to the fourth embodiment isformed in a manner as described below.

(Underlying Layer 302)

The underlying layer 302 is, for example, made of GaN and has athickness of 4 μm. The underlying layer 302 is formed via a200-angstrom-thick buffer layer (not shown) consisting of GaN on anauxiliary substrate 301 made of, for example, sapphire. The underlyinglayer 302 forms partially a protective film on the surface and is usedfor the layer on which the nitride semiconductor substrate is to begrown selectively. The underlying layer 302 is made of GaN orAl_(X)Ga_(1-X)N and in the case of including Al, Al_(X)Ga_(1-X)N(0≦X≦0.5) having a Al mixed crystal ratio exceeds 0.5, cracking easilyoccurs in the crystal itself, rather than the crystal defects, resultingin difficulty of the crystal growing itself. The under lying layer 302is desirably grown to a larger thickness than that of the buffer layerand adjusted to not more than 10 μm. The auxiliary substrate 301 may bemade of SiC, ZnO, spinel, GaAs and the like, as well as sapphire, whichare known to be used for growing the nitride semiconductors and aredifferent from nitride semiconductors.

(Protective Film 303)

For a protective film 303, 1-μm-thick SiO₂ film having a sufficientlength in the resonating direction is patterned to form 10-μm-widestripe windows with a periodicity of 2 μm on the underlying layer 302.The examples of the protective film may include stripes, dots or checks.The protective film 303 may preferably have a larger area than thewindows (the exposed parts of the underlying layer 302 on which the SiO₂is not formed), so as to grow a nitride semiconductor substrate 304 withless crystal defects. The materials of the protective film 303 mayinclude oxides such as silicon oxide (SiO_(x)), silicon nitride(Si_(x)Ni_(y)), titanium oxide (TiO_(x)) and zirconium oxide (ZrO_(x)),nitrides, or multi-layer film thereof, as well as metals having amelting point higher than 1200° C. These materials can stand at hightemperatures ranging between 600° C. and 1100° C. at which nitridesemiconductors can be grown and nitride semiconductors do not grow orare difficult to grow on the surface of the materials.

(Nitride Semiconductor Substrate 304)

For a nitride semiconductor substrate 304, for example, undoped GaNhaving a thickness of, for example, 20 μm is grown on the protectivefilm 303 using MOVPE method. The nitride semiconductor substrate 304 maybe grown using Halide Vapor Phase Epitaxy (HVPE) method as well as usingMOVPE method in this way. The nitride semiconductor substrate may bemost preferably obtained by growing GaN without In or Al. It is mostdesirable to use TMG as well as organic gallium compounds such astriethylgallium (TEG) as a gas for growth and to use ammonia orhydrazine as nitrogen sources. Donor impurities such as Si and Ge may bedoped to the GaN substrate so as to adjust the carrier density to thesuitable range. In the case that the auxiliary substrate 301, theunderlying layer 302, and the protective film 303 are eliminated, thenitride semiconductor substrate functions as a contact layer andtherefore, donor impurities may be desirably doped to the nitridesemiconductor substrate.

(n-Side Buffer Layer 311=which Also Functions as a Contact Layer)

The n-side buffer layer 311 is obtained, for example, by forming a 5μm-thick GaN doped with Si to 3×10¹⁸/cm³ on the nitride semiconductorsubstrate 304. The buffer layer 311 functions as a contact layer forforming a n-electrode in the case of the fabrication of the lightemitting device having a structure as shown in FIG. 9. In the case thatthe auxiliary substrate 301 to the protective film 303 are eliminatedand the electrode is formed on the nitride semiconductor substrate 304,the n-side buffer layer can be omitted. The n-side buffer layer 311 isone which is grown at high temperature and is different from the bufferlayer which is formed by growing GaN, AlN and the like to a thickness of0.5 μm or less directly on the substrate of the material such assapphire, SiC and spinel and which is different from the nitridesemiconductor at a low temperature of 900° C. or less.

(Crack-Preventing Layer 312)

The crack-preventing layer 312 is formed by, for example, growingIn_(0.06)Ga_(0.84)N to a thickness of 0.15 μm on the n-side buffer layer311.

(n-Side Cladding Layer 313=Super Lattice Layer)

The n-side cladding layer 313 is composed of a super lattice layer whichis obtained by laminating alternately the 25-angstrom-thick first layerconsisting of n-type Al_(0.16)Ga_(0.84)N doped with Si to 1×10¹⁸/cm³ andthe 25-angstrom-thick second layer consisting of undoped GaN, and has atotal thickness of, for example, 1.2 μm. The n-side cladding layer 313composed of a super lattice layer has an average Al composition of 8.0%and the product of that and the film thickness equals 9.6. In the casethat the n-side cladding layer 313 is composed of a super lattice layerobtained by laminating the nitride semiconductor layers which havedifferent band gap energy from each other, more impurities may be dopedto one layer, that is, modulation-doping may be performed, so as tolower the threshold value. The composition and thickness of the n-sidecladding layer 313 (a super lattice layer) will be described in detailbelow.

(n-Side Light Waveguide Layer 314)

The n-side light waveguide layer 314 is made of undoped GaN and has athickness of, for example, 0.1 μm. The n-side light waveguide layerfunctions as a light waveguide layer for the active layer. The n-sidelight waveguide layer is desirably formed by growing GaN or InGaN to athickness, usually ranging between 100 angstroms and 5 μm, preferablyranging between 200 angstroms and 1 μm.

(Active Layer 315)

The active layer 315 is made by alternately laminating a quantum welllayer which is made of undoped In_(0.2)Ga_(0.8)N and has a thickness of40 angstroms and a barrier layer made of undoped In_(0.01)Ga_(0.99)N andhas a thickness of 100 angstroms, thereby forming a layer of multiplequantum well structure having a total thickness of, for example, 440angstroms. The active layer 315 may be undoped as in the presentembodiment or may be doped with donor impurities and/or p-typeimpurities. In this case, either both or one of the quantum well layerand the barrier layer may be doped with the impurity.

(p-Side Cap Layer 316)

The p-side cap layer 316 has band gap energy higher than that of thep-side light waveguide layer 317 which is formed thereon, and is made ofp-type Al_(0.3)Ga_(0.7)N doped with Mg in a concentration of 1×10²⁰/cm³and has a thickness of, for example, 300 angstroms. The p-type cap layer316 has a thickness of 0.1 μm or less so as to enhance the laser of thedevice. The lower limit of the film thickness is not specified, but thep-side cap layer 316 may desirably have a thickness of 10 angstroms ormore.

(p-Side Light Waveguide Layer 317)

The p-side light waveguide layer 317 has band gap energy lower than thatof the p-side cap layer 316, and is made of, for example, undoped GaNand has a thickness of 0.1 μm. The p-side light waveguide layerfunctions as a light waveguide layer for the active layer and isdesirably made of GaN or InGaN like the n-type light waveguide layer314.

(p-Side Cladding Layer 318)

The p-side cladding layer 318 has a super lattice structure made byalternately laminating the third layer which is made of p-typeAl_(0.16)Ga_(0.84)N doped with Mg to 1×10²⁰/cm³ and has a thickness of25 angstroms and the fourth layer which is made of undoped GaN and has athickness of 25 angstroms. The p-side cladding layer 318 has an averageAl composition of 8% and the product of the value and the film thicknessequals 4.8. In the case that the p-side cladding layer 318 is composedof a super lattice layer obtained by laminating the nitridesemiconductor layers at least one of which includes a nitridesemiconductor layer containing Al and which have different band gapenergy from each other, more impurities may be doped to any one layer,that is, modulation-doping may be performed, so as to lower thethreshold value. The composition and thickness of the p-side claddinglayer 318 (a super lattice layer) will be described in detail below.

Now, the thickness of the core part (waveguide part) which is sandwichedbetween the cladding layers will be described below. The core part isthe area including the n-side light waveguide layer 314, active layer315, p-side cap layer 316 and the p-side light waveguide layer 317, thatis, the nitride semiconductor layers including the active layer whichare sandwiched between the n-side cladding layer and the p-side claddinglayer and the area which waveguides the emitting from the active layer.In the case of the nitride semiconductor laser device, as mentionedabove, FFP is not a single beam because the emission leaking via thecladding layer can be wave-guided within the n-side contact layer,resulting in multi-mode. Otherwise, the emission is resonated within thecore part, resulting in multi-mode. According to the present inventionsthe n-side cladding layer has a large thickness and a large average Alcomposition in order to obtain the difference between the refractiveindices and to trap the emission from the core part within the claddinglayer. However, if the multi-mode occurs within the core part, FFP isdisturbed. Therefore, in order that the multi-mode does not occur withinthe core part, the thickness of the core part is desirably adjusted,with respect to the n-side cladding layer. The thickness is desirablyadjusted to the range from 200 angstrom to 1.0 μm, preferably from 500angstroms to 0.8 μm, most preferably from 0.1 μm to 0.5 μm, in orderthat the multi-mode does not occur within the core part. If thethickness is below 200 angstroms, the emission leaks from the core partto increase the threshold value. If the thickness is above 1.0 μm, themulti-mode is likely to occur.

(p-Side Contact Layer 319)

The p-side contact layer 319 is made of, for example, p-type GaN dopedwith Mg to 2×10²⁰/cm³ and has a thickness of, for example, 150 angstrom.The p-side contact layer 319 can be made in a constitution of p-typeIn_(X)A_(Y)Ga_(1-X-Y)N (0≦x, 0≦Y, X+Y≦1), as well as p-type GaN asmentioned above, and the preferable ohmic contact with the p electrode321 can be obtained by using GaN doped with Mg.

In the embodiment 4, the wafer on which each nitride semiconductor layeris formed is preferably annealed at 700° C. within the nitrogenatmosphere in the reactor, so as to decrease the resistance of the layerdoped with the p-type impurity.

In the laser device of the embodiment 4, the p-side contact layer 318which is a top layer, and the p-side cladding layer are etched with theRIE apparatus to form a ridge geometry having a stripe width of 4 μm, asshown in FIG. 9. When the ridge stripe is formed, the ridge stripe isformed above the position of the surface of the nitride semiconductorsubstrate 304 where the crystal defects do not appear. In FIG. 9, thecrystal defects appear in the central part of the protective film 303 inthe form of stripe and in the central part of the window in the form ofstripe. Thus, when the stripe is formed at the point where almost nocrystal defects are present, the crystal is prevented from developing tothe active layer, with the result that the laser device can has a longlifetime and the reliability thereof can be enhanced.

Further, a mask is formed on the ridge surface and the etching isperformed with RIE to expose the surface of the n-side buffer layer 311on both sides of the ridge. On the exposed surface of the n-side bufferlayer 311, the n electrode 322 made of Ti and Al is formed,respectively.

And on the outermost surface of the ridge of the p-side contact layer319, the p electrode 320 made of Ni and Au is formed in the form ofstripe. As shown in FIG. 9, the insulating film 323 made of SiO₂ isformed on the surface of the nitride semiconductor layer which isexposed between the p electrode 320 and the n electrode 322. The p-padelectrode 321 which is electrically connected to the p electrode 320 isformed via the insulating film 323.

The sapphire substrate of the wafer on which the n electrode and pelectrode are formed in a manner as mentioned above is polished to athickness of 70 μm and cleaved into bars from the substrate,perpendicularly with respect to the stripe-shaped electrode to fabricatea resonator in which the cleaving facet functions as a reflective plane.A multi-layer dielectric film made of SiO₂ and TiO₂ may be formed on thereflective plane.

The laser device of the fourth embodiment is fabricated as describedabove.

The laser device of the embodiment 4 fabricated as descibed above has asa light trapping layer the n-side cladding layer 313 and the p-sidecladding layer 318 of a super lattice structure which have smallerrefractive indices than the well layer of the active layer and includenitride semiconductor, respectively. In the embodiment 4, the supperlattice layer means the multi-layer structure obtained by laminatingnitride semiconductor layers which have a thickness of not more than 100angstroms respectively and have a different composition from each other.The thickness of the layer which is laminated is preferably not morethan 70 angstroms, and more preferably not more than 40 angstroms. To beconcrete, for example, the super lattice layer may be made by laminatingthe layer made of Al_(X)Ga_(1-X)N (0<X<1) and the other nitridesemiconductor layer which has a different composition from the layermade of Al_(X)Ga_(1-X)N, and by laminating the layer of the ternarymixed crystal and the layer of the ternary mixed crystal or the layer ofthe ternary mixed crystal and the layer of binary mixed crystal such asAl_(X)Ga_(1-X)N/GaN, Al_(X)Ga_(1-X)N/Al_(Y)Ga_(1-Y)N (0<Y<1, Y<X),Al_(X)Ga_(1-X)N/In_(Z)Ga_(1-Z)N (0<Z<1) and so on. Thereamong, the mostpreferable super lattice layer is made of Al_(X)Ga_(1-X)N and GaN.

Next, the total thickness of the n-side cladding layer 313 and thethickness and the composition of each layer which constitute the superlattice layer according to the embodiment 4 will be described.

First, in these specifications, the Al average composition of the superlattice layer means that calculated as described below. For example,when the super lattice layer is made by laminating a 25-angstrom-thickAl_(0.5)Ga_(0.5)N and a 25-angstrom-thick GaN in 200 pairs (1.0 μm), thethickness of one pair is 50 angstroms and the mixing proportion of Al ofthe layer containing Al is 0.5. Using the value 0.25 obtained bymultiplying the mixing ratio of Al of the layer containing Al, 0.5, bythe film thickness ratio (25/50), the Al average composition of thesuper lattice layer is 25%.

When the super lattice layer is made by laminating the layers having adifferent thickness, that is, by laminating a 40-angstrom-thickAl_(0.5)Ga_(0.5)N and a 20-angstrom-thick GaN, the weighed mean of thefilm thickness is calculated to be 0.5 (40/60)=0.33, and therefore, theAl average composition is 33.3. That is, the mixing ratio of Al of thenitride semiconductor layer containing Al multiplied by the ratio ofsaid nitride semiconductor layer to one pair of the super lattice layersequals the Al average composition in the present invention. When bothlayers contain Al, the Al average composition can be obtained in thesame manner. For example, the super lattice layer is made by laminatinga 20-angstrom-thick Al_(0.1)Ga_(0.8)N and a 30-angstrom-thickAl_(0.2)Ga_(0.8)N, 0.1 (20/50)+0.2 (30/50)=0.16, that is, the Al averagecomposition is 16%. The example mentioned above refers to AlGaN/GaN andAlGaN/AlGaN and the same calculation method can be applied to the caseof AlGaN/InGaN. And, the Al average composition of the n-side claddinglayer can be detected with an instrument for analysis such as SIMS(secondary ion mass spectrometer) and Auger electron spectrometer.

In the embodiment 4, the composition and the film thickness of eachlayer which constitute the super lattice layer of the n-side claddinglayer 313 is set on the basis of the Al average composition ascalculated using the above-mentioned calculating method. The laserdevice according to the embodiment 4 is characterized in that the totalthickness of the n-side cladding layer 313 is not less than 0.5 μm andthe Al average composition in % of the n-side cladding layer 313 is setto be such that the product of the Al average composition (%) multipliedby the total thickness of the n-side cladding layer (μm) is not lessthan 4.4. In other words, in the embodiment 4, the thickness and the Alaverage composition of each layer constituting the super lattice layerare set to be such that the total thickness of the n-side cladding layer313 is not less than 0.5 μm and the product of said thickness multipliedby the Al average composition in % as calculated in the above-mentionedway is not less than 4.4.

When the total thickness of the n-side cladding layer 313 is less than0.5 μm and the product of said total thickness (μm) multiplied by the Alaverage composition (%) is less than 4.4, the light trapping effect ofthe n-side cladding layer is insufficient and the resonance occurs againin the n-side contact layer, with the result that the FFP is disturbedand the threshold value tends to increase. The product of the Al averagecomposition multiplied by the total thickness of the n-side claddinglayer 313 is preferably not less than 5.0, more preferably not less than5.4, most preferably not less than 7.0.

According to the present invention, because of the n-side cladding layer313 composed of a super lattice layer, even if the mixing ratio of Al isincreased, the cracks can be difficult to develop in the cladding layer.Therefore, the upper limit of the total thickness of the n-side claddinglayer 313 is not specified from the viewpoint of crystralinity (from theviewpoint in which the cracks are prevented from occurring), andhowever, it is desirable to control the thickness to be not more than 5μm so as to decrease the laminating times of the nitride semiconductorlayers which constitute the super lattice layer.

To be concrete, for example, the total thickness of said n-side claddinglayer is set to be not less than 0.8 μm and the Al average compositionwhich is contained in said n-side cladding layer is set to be not lessthan 5.5%. In this case, the product is not less than 4.4. Preferably,the total thickness of said n-side cladding layer is set to be not lessthan 1.0 μm and the Al average composition which is contained in saidn-side cladding layer is set to be not less than 5.0%. In this case, theproduct is not less than 5.0. More preferably, the total thickness ofsaid n-side cladding layer is set to be not less than 1.2 μm and the Alaverage composition which is contained in said n-side cladding layer isset to be not less than 4.5%. In this case, the product is not less than5.4. The examples mentioned above described concretely the relationbetween the thickness of the n-side cladding layer and the Al averagecomposition of the n-side cladding layer made in super latticestructure. It is known that when the mixing ratio of Al inAl_(X)Ga_(1-X)N is increased, the band gap energy increases and therefractive index decreases. Therefore, it is ideal and industriallyconvenient that the Al_(X)Ga_(1-X)N layer having a large mixing ratio ofAl of, for example, not less than 0.5 is grown in a single layer of, forexample, a few μm, however, the Al_(X)Ga_(1-X)N layer is difficult to begrown thickly. If the Al_(X)Ga_(1-X)N, particularly having a mixingratio of Al of not less than 0.5, is intended to be grown in a singlelayer, the cracks should develop in the crystal in the thickness of, forexample, not less than 0.1 μm. Thus, the Al_(X)Ga_(1-X)N layer having amixing ratio of Al of not less than 0.5 is difficult to be grown in asingle layer having a thickness of, for example, a few μm.

However, according to the present invention, a thin film made ofAl_(X)Ga_(1-X)N is used to constitute a super lattice layer and thethickness of the single film is not more than the critical filmthickness of Al_(X)Ga_(1-X)N, therefore, the cracks being to develop.Therefore, when the cladding layer is composed of a super lattice layer,the layer having a high mixing ratio of Al and a large thickness can begrown. In the present invention, since the relation between the specificmixing ratio of Al and the thickness of the cladding layer could befound, the combination thereof enables the light not to leak from then-side cladding layer to the substrate side.

When the n-side cladding layer is constituted as mentioned above to trapthe emission from the active layer, the -side cladding layer may havethe same constitution as that of the n-side cladding layer. When thep-side cladding layer 318 is constituted in the same manner as then-side cladding layer 313, the thickness of the p-side cladding layer isdesirably smaller than that of the n-side cladding layer. Because, whenthe mixing ratio of Al or the thickness of the p-side cladding layer islarge, the resistance of the AlGaN layer tends to increase with theresult that the threshold value tends to increase. Therefore, even ifthe p-side cladding layer is constituted of a super lattice layerincluding the nitride semiconductor layer containing Al and the productof the thickness multiplied by the Al average composition is not lessthan 4.4, the thickness is desirably no more than 1.0 μm. The lowerlimit is not specified but is desirably not less than 50 angstroms inorder to function as a cladding layer. In the case of the super latticelayer, the Al average composition is desirably not more than 50%. Sincethe p-side cladding layer is shaped into a ridge and the electrode isinstalled thereon, the leakage of the light can be almost neglected andit is not necessary for the p-side cladding layer to have the sameconstitution as that of the n-side cladding layer 313, but the p-sidecladding layer may have the same constitution as that of the n-sidecladding layer. That is, the p-side cladding layer has a super latticestructure including the nitride semiconductor layer containing at leastAl and has a total thickness of not more than 1.0 μm. Moreover, the Alaverage composition in % contained in the p-side cladding layer may beset to be such that the product of the total thickness (μm) of thep-side cladding layer multiplied by the Al average composition (%) isnot less than 4.4.

When the p-side cladding layer is composed of a super lattice layerincluding the nitride semiconductor layer containing Al (in this case,the leakage of light is not concerned and the case in which the claddinglayer functions only as a carrier trapping layer is included), the totalthickness of the n-side cladding layer is desirably larger than that ofthe p-side cladding layer. The p-side cladding layer is composed of asuper lattice layer made by laminating the nitride semiconductor layersin the same way as the n-side cladding layer, for example, by laminatingthe Al_(X)Ga_(1-X)N (0<X<1) layer and the other nitride semiconductorlayer which has a different composition from said Al_(X)Ga_(1-X)N layer,or by laminating the layer made of ternary mixed crystal and the layermade of ternary mixed crystal or the layer made of ternary mixed crystalant the layer made of binary mixed crystal such as Al_(X)Ga_(1-X)N/GaN,Al_(X)Ga_(1-X)N/Al_(Y)Ga_(1-Y)N (0≦Y≦1, Y<X),Al_(X)Ga_(1-X)N/In_(Z)Ga_(1-Z)N (0<Z<1) and so on. Thereamong, the mostpreferable super lattice layer is made of Al_(X)Ga_(1-X)N and GaN.

The present invention will be described in detail in the followingexamples.

EXAMPLES

The invention will be described in detail in the following examples.

Example 1

According to Example 1 of the invention, nitride semiconductor devices(LD devices), as shown in FIGS. 1 and 2, can be fabricated. The nitridesemiconductor devices were fabricated as follows.

First, a C-plane sapphire substrate 10 was set in the reactor and theinside atmosphere of the reactor was fully replaced with hydrogen. Thetemperature of the substrate was increased to 1050° C. with hydrogenflown in order to clean the substrate.

Subsequently, the temperature was decreased to 510° C. A first bufferlayer 11 consisting of GaN was grown to a film thickness of about 200angstrom using hydrogen as a carrier gas, ammonia and TMG(trimethylgallium) as a source of GaN.

After growing the buffer layer, only TMG was stopped and the temperaturewas increased to 1050° C. At 1050° C. in the same way using ammonia andTMG (trimethylgallium) as a source of GaN, a 5 μm-thick second bufferlayer 112 consisting of undoped GaN having a carrier density of1×10¹⁸/cm³ was grown.

Subsequently, a 1 μm-thick n-side contact layer 12 consisting of n-typeGaN doped with Si to 1×10¹⁹/cm³ was grown, using TMG and ammonia andsilane gas (SiH₄) as a source of impurity at 1050° C.

Next, the temperature was increased to 800° C. A 500-angstrom-thickcrack-preventing layer consisting In_(0.1)Ga_(0.9)N doped with Si to5×10¹⁸/cm³ was grown, using TMG, TMI (trimethylindium) and ammonia assource gases and silane gas as a source of impurity.

Then, the temperature was increased to 1050° C. and a 20-angstrom-thickfirst layer consisting of n-type Al_(0.2)Ga_(0.8)N doped with Si to5×10¹⁸/cm³ was grown, using TMA, TMG, ammonia and silane gas.Subsequently, TMA and silane were stopped and a 20-angstrom-thick secondlayer consisting of undoped GaN was grown. Then, each of theseoperations was repeated 100 times and a n-side cladding layer 14 ofsuperlattices having a total thickness of 0.4 μm was grown.

Subsequently, a 0.1 μm-thick n-side light guide layer 15 consisting ofn-type GaN doped with Si to 5×10¹⁸/cm³ was grown at 1050° C.

Next, an active layer 16 was grown using TMG, TMI, ammonia and silane.For the active layer 16, first, a 25-angstrom-thick well layer 25consisting of In_(0.2)Ga_(0.8)N doped with Si to 8×10¹¹/cm³ was grown at800° C. Next, at the same temperature, a 50-angstrom-thick barrier layerconsisting of In_(0.01)Ga_(0.99)N doped with Si to 8×10¹⁸/cm³ was grown,only by changing the molar ratio of TMI. This operation was repeatedtwice. Finally, the well layer was laminated and amulti-quantum-well-structure active layer 16 was grown with a totalthickness of 175 angstroms.

Next, the temperature was increased to 1050° C. A 300-angstrom-thickp-side cap layer 17 consisting of p-type Al_(0.3)Ga_(0.7)N doped with Mgto 1×10²⁰/cm³ was grown which had a greater band gap energy than theactive layer, using TMG, TMA and ammonia as a source of AlGaN and Cp2Mg(cyclopentadienyl magnesium) as a source of impurity.

Subsequently, a 0.1 μm-thick p-side light guide layer 18 consisting ofp-type GaN doped with Mg to 1×10²⁰/cm³ was grown which had a smallerband gap energy than the p-side cap layer 17 at 1050° C.

Subsequently, a 20-angstrom-thick first layer consisting of p-typeAl_(0.2)Ga_(0.8)N doped with Mg to 1×10²⁰/cm³ was grown, using TMA, TMG,ammonia and Cp2Mg at 1050° C. Subsequently, TMG was stopped and a20-angstrom-thick second layer consisting of p-type GaN doped with Mg to1×10²⁰/cm³ was grown. Each of these operations was repeated 100 timesand a p-side cladding layer 19 of superlattices with a total thicknessof 0.4 μm was formed.

Finally, a 150-angstrom-thick p-side contact layer 20 consisting ofp-GaN doped with Mg to 2×10²⁰/cm³ was grown on the p-side cladding layer19 at 1050° C.

After the reaction was completed, the temperature was decreased to roomtemperature. Additionally, the annealing was performed to the wafer at700° C. in nitrogen atmosphere within the reactor, so as to make thep-type layer less resistive. The annealing method disclosed in U.S. Pat.No. 5,306,662 is employed in this Example.

After annealing, the wafer was removed from the reactor. As shown inFIG. 2, the top p-side contact layer 20 and the p-side cladding layer 19were etched with RIE apparatus to make them a ridge geometry having astripe width of 4 μm.

Next, a mask was formed on the surface of the ridge. As shown in FIG. 2,the surfaces of the n-side contact layer were exposed symmetricallyrespect to the stripe ridge.

Next, a p-electrode 21 consisting of Ni and Au was formed on the almostwhole top surface of the stripe ridge of the p-side contact layer 20,while a n-electrode 23 consisting of Ti and Al was formed on the almostwhole surface of the stripe-geometry n-side contact layer.

Next, as shown in FIG. 2, an insulating film 25 was formed on thesurface of the nitride semiconductor layer exposed between p-electrode21 and n electrode 23. A p-pad electrode 22 and an n-pad electrode 24were formed which are electrically connected to the p-electrode via theinsulating film 25.

The wafer on which the n-electrode and p-electrode were formed in theabove-mentioned manner was transferred to the polishing machine. Thesapphire substrate on whose side the nitride semiconductor was notformed was lapped with a diamond abrasive to a substrate thickness of 50μm. After lapping, the surface of the substrate was further polished by1 μm with a finer abrasive, resulting in the mirror facet.

After polishing the substrate, the polished facet was scribed andcleaved into bars perpendicularly respect to the stripe-geometryelectrode to fabricate a facet of a resonator on the cleaving facet. Adielectric multi-layer film consisting SiO₂ and TiO₂ was formed on thefacet of the resonator and finally, the bar was cut parallel to thep-electrode, resulting in laser chips. Next, the chips were set face-up(in the state that the substrate was faced to the heat sink) onto theheat sink and each electrode was connected by wire-bonding. The laseroscillation was tried at room temperature. The continuous emission at anemission wavelength of 405 nm was observed at the threshold currentdensity of 2.9 kA/cm² and the threshold voltage of 4.4V at roomtemperature. The lifetime was 50 hours or longer.

Comparative Example 1

With the same procedure as in Example 1, the second buffer layer was notgrown, the 5 μm-thick n-side contact layer 12 only consisting of n-typeGaN doped with Si to 1×10¹⁹/cm³ was grown, the 0.4 μm-thick n-sidecladding layer only consisting of n-type Al_(0.2)Ga_(0.8)N doped with Sito 1×10¹⁹/cm³ was grown, the 0.4 μm-thick p-side cladding layer 19 onlyconsisting of p-type Al_(0.2)Ga_(0.8)N doped with Mg to 1×10²⁰/cm³ wasgrown, and the 0.2 μm-thick p-side contact layer 20 only consisting ofp-type GaN doped with Mg to 2×10²⁰/cm³ was grown, so as to obtain laserdevices. The basic construction was as shown in FIG. 1.

TABLE 1 Substrate 10 sapphire Buffer layer 11 GaN 200 Å n-side contactlayer 12 Si doped n-type GaN 5 μm Si: 1 × 10¹⁹/cm³ Crack preventinglayer 13 Si doped n-type In_(0.1)Ga_(0.9)N 500 Å Si: 5 × 10¹⁸/cm³ n-sidecladding layer 14 Si doped n-type Al_(0.2)Ga_(0.8)N 0.5 μm Si: 5 × 10¹⁸cm³ n-side optical 15 Si doped n-type GaN 0.1 μm waveguide layer Si: 5 ×10¹⁸/cm³ Active layer (MQW) 16 Si doped In_(0.2)Ga_(0.8)N 25 Å (totalthickness 175 Å) Si doped In_(0.01)Ga_(0.95)N 50 Å Si: 8 × 10¹⁸ cm³ Caplayer 17 Mg doped p-type Al_(0.1)Ga_(0.9)N 300 Å Mg: 1 × 10²⁰ cm³ p-sideoptical 18 Mg doped p-type GaN 0.1 μm waveguide layer Mg: 1 × 10²⁰ cm³p-side cladding layer 19 Mg doped p-type Al_(0.2)Ga_(0.8)N 0.5 μm Mg: 1× 10²⁰ cm³ p-side contact layer 20 Mg doped p-type GaN 0.2 μm Mg: 2 ×10²⁰ cm³

For the laser devices of Comparative Example 1 constructed in this way,the continuous emission was obtained at the threshold current density of7 kA/cm². However, the threshold voltage was 8.0V or higher and theemission was observed for few minutes.

Example 2

With the same procedure as in Example 1, for the n-side contact layer12, a 30-angstrom-thick first layer consisting of n-typeAl_(0.05)Ga_(0.95)N doped with Si to 2×10¹⁹/cm³ was grown andsubsequently, a 30-angstrom-thick second layer consisting of undoped GaNwas grown and these procedures were repeated, resulting in superlatticeshaving a total thickness of 1.2 μm. The other constructions of the laserdevices were the same as in Example 1. The threshold current density was2.7 kA/cm², the threshold voltage was 4.2V and the lifetime was 60 hoursor longer.

Example 3

The laser devices were fabricated which had the same constructions as inExample 2, except for the n-side contact layer 12 composed ofsuperlattices whose second layers consisted of GaN doped with Si to1×10¹⁸/cm³. The laser devices had almost similar properties to those ofExample 2.

Example 4

The laser devices were fabricated which had the same constructions as inExample 1, except that the 4-thick second buffer layer 112 consisting ofGaN doped with Si to 1×10¹⁷/cm³ was grown. The threshold current densitywas 2.9 kA/cm², the threshold voltage was 4.5V and the lifetime was 50hours or longer.

Example 5

With the same procedure as in Example 1, for the n-side contact layer12, a 60-angstrom-thick first layer consisting of n-typeAl_(0.2)Ga_(0.8)N doped with Si to 2×10¹⁹/cm³ was grown andsubsequently, a 40-angstrom-thick second layer consisting of GaN dopedwith Si to 1×10¹⁹/cm³ of Si was grown and these procedures wererepeated, resulting in superlattices having a total thickness of 2 μm.And the 0.4 μm-thick n-side cladding layer 14 consisting of only n-typeAl_(0.2)Ga_(0.8)N doped with Si to 1×10¹⁹/cm³ was grown. The otherconstructions of the laser devices were the same as those in Example 1.The threshold current density was 3.2 kA/cm², the threshold voltage was4.8V and the lifetime was 30 hours or longer.

Example 6

With the same procedure as in Example 1, the following procedures (1)and (2) were changed.

(1) After the growth of the buffer layer 11, only TMG was stopped andthe temperature was increased to 1050° C. At 1050° C., the60-angstrom-thick first layer consisting of n-type Al_(0.2)Ga_(0.8)Ndoped with Si to 1×10¹⁹/cm³ was grown using TMA, TMG, ammonia and silaneas material gases and subsequently, silane and TMA were stopped and the60-angstrom-thick second layer consisting of undoped GaN was grown. Thesuperlattices were constructed in a manner of the first layer+the secondlayer+the first layer+ the second layer+ . . . . Five-hundreds firstlayers and five-hundred second layers were laminated by turns. Then-side contact layer 12 of superlattices having a total thickness of 5μm was formed.

(2) Next, the 500-angstrom thick-crack preventing layer 13 consisting ofIn_(0.1)Ga_(0.9)N doped with Si to 5×10¹⁸/cm³ was grown in the samemanner as in Example 1.

And at 1050° C. the 0.5 μm-thick n-side cladding layer 14 consisting ofn-type Al_(0.2)Ga^(0.8)N doped with Si to 5×10¹⁸/cm³ was grown usingTMA, TMG, ammonia and silane.

The constructions above the n-side cladding layer were the same as thoseof the laser devices in Example 1. The laser devices of this example hadthe basic constructions as described in Table 1, except that the n-sidecontact layer 12 and p-side cladding layer 19 were composed ofsuperlattices and the p-side contact layer 20 had a thickness of 150angstrom like Example 1. The continuous emission at a wavelength of 405nm was observed at the threshold current density of 3.2 kA/cm² and thethreshold voltage of 4.8V. The lifetime was 30 hours or longer.

Further, in the case that the film thickness of the p-side contact layerof LDs according to Example 6 is changed gradually, the relation betweenthe film thickness of the p-side contact layer and the threshold voltageof the LDs is shown in FIG. 5. The figure shows the threshold voltagesin the case the p-side contact layer has a thickness of less than 10angstrom (A), 10 angstrom (B), C (30 angstrom), 150 angstrom (D: thisexample), 500 angstrom (E), 0.2 μm (F), 0.5 μm (G) and 0.8 μm (H), inorder from the left. As shown in this drawing, the threshold voltagetends to increase gradually when the film thickness of the p-sidecontact layer is over 500 angstrom. The p-side contact layer 20preferably has a thickness of 500 angstrom or less, more preferably 300angstrom or less. When the film thickness is 10 angstrom or less (almostone atom layer or two atom layer), the surface of the p-side claddinglayer 19 which is under the p-side contact layer is exposed andtherefore, the contact resistance of the p-electrode is bad, resultingin that the threshold voltage tends to increase. However, since LDs ofthe present invention had a superlattice layer, the threshold voltagewas much lower than that of the comparative examples.

Comparative Example 2

The laser devices having the construction as described in Table 1 werefabricated, except that n-side cladding layer 14 was formed which was amulti-layer film having a total thickness of 0.6 μm, by growing the180-angstrom-thick first layer consisting n-type Al_(0.2)Ga_(0.8)N dopedwith Si to 1×10¹⁹/cm² and subsequently, growing the 120-angstrom-thicksecond layer consisting of undoped GaN. That is, the thickness of thefirst layer and second layer was increased and LDs were fabricated. Thecontinuous emission was observed at the threshold current density of 6.5kA/cm² and the threshold voltage was 7.5 V. The emission was observedfor few minutes.

Example 7

The laser devices having the same construction as in Example 6 werefabricated, except that the p-side cladding layer 19 had a superlatticestructure with a total thickness of 0.5 μM obtained by laminating the60-angstrom-thick first layers consisting of Al_(0.2)Ga_(0.8)N dopedwith Mg to 1×10²⁰/cm³ and the 40-angstrom-thick second layers consistingof p-type GaN doped with Mg to 1×10²⁰/cm³. That is, the thickness of thesuperlattice layer constructing the p-side cladding layer 19 of Example7 was changed and the other constructions were the same as in Example 7.The threshold voltage increased a little, as compared with that inExample 6. The lifetime was 20 hours or longer.

Example 8

The laser devices having the same construction as in Example 7 werefabricated, except that the n-side cladding layer 19 had a superlatticestructure with a total thickness of 0.5 g m obtained by laminating the60-angstrom-thick first layers consisting of Al_(0.2)Ga_(0.8)N dopedwith Si to 1×10¹⁹/cm³ and the 40-angstrom-thick second layers consistingof n-type GaN doped with Si to 1×10¹⁹/cm³. That is, the laser deviceshad superlattices as the n-side cladding layer, in addition to then-side contact layer and p-side cladding layer having a superlatticestructure in Example 6. The present laser devices had similar propertiesto those of Example 6.

Example 9

With the same procedures as in Example 1, the second buffer layer 112was not grown and the 5 μm-thick n-side contact layer consisting ofn-type GaN doped with Si to 1×10¹⁹/cm³ was grown directly on the firstbuffer layer 11. The other constructions were the same as those inExample 1. That is, the laser devises had the basic constructions asdescribed in Table 1, except that the n-side cladding layer 14 had asuperlattice structure with a total thickness of 0.4 μm obtained bylaminating the 20-angstrom-thick first layers consisting of n-typeAl_(0.2)Ga_(0.9)N doped with Si to 1×10¹⁹/cm³ and the 20-angstrom-thicksecond layers consisting of undoped GaN. Further, the p-side claddinglayer had a superlattice structure with a total thickness of 0.4 μmobtained by laminating the 20-angstrom-thick first layers consisting ofp-type Al_(0.2)Ga_(0.8)N doped with Mg to 1×10¹⁹/cm³ and the20-angstrom-thick second layers consisting of p-type GaN doped with Mgto 1×10²⁰/cm³. Additionally, the p-side contact layer 20 consisted ofp-type GaN doped with Mg to 2×10²⁰/cm³ and had a thickness of 15angstrom. The continuous emission at a wavelength of 405 nm was observedat the threshold current density of 3.3 kA/cm² and the threshold voltagewas 5.0V. The lifetime was 30 hours or longer.

Example 10

The laser devices were fabricated in the same manner as in Example 9,except that the second layer which constructed the superlattices of then-side cladding layer 14 consisted of GaN doped with Si to 1×10¹⁷/cm³.That is, the laser devices were fabricated in the same manner as inExample 9, except that the layers having a larger band gap energy weredoped with Si to more amount. The present laser devises had similarproperties to those of Example 9.

Example 11

The laser devices were fabricated in the same manner as in Example 9,except that the second layer which constructed the n-side cladding layer14 consisted of n-type In_(0.01)Ga_(0.99)N doped with Si to 1×10¹⁹/cm³.That is, the laser devices were fabricated in the same manner as inExample 9, except that the second layer which composed the superlatticesof the n-side cladding layer 14 consisted of InGaN and had the sameimpurity density as the first layer. The present laser devises hadsimilar properties to those of Example 9.

Example 12

With the same procedures as in Example 9, the n-side cladding layer hada superlattice structure with a total thickness of 0.5μ composed of the60-angstrom-thick first layers consisting Al_(0.2)Ga_(0.8)N doped withSi to 1×10¹⁹/cm³ and the 40-angstrom-thick second layers consisting ofGaN doped with Si to 1×10¹⁹/cm³. Further, the p-side cladding layer 19had a superlattice structure with a total thickness of 0.5μ composed ofthe 0.60-angstrom-thick first layers consisting Al_(0.2)Ga_(0.8)N dopedwith Mg to 1×10²⁰/cm³ and the 40-angstrom-thick second layers consistingof GaN doped with Mg to 1×10²⁰/cm³. That is, the laser devices werefabricated in the same manner as in Example 9, except that the dopedamounts in the first layers and the second layers composing the n-sidecladding layer 14 were equal to each other, the thickness of them waschanged, and the thickness of the first layers and the second layerscomposing the p-side cladding layer 19 was changed. The continuousemission at a wavelength of 405 nm was observed at the threshold currentdensity of 3.4 kA/cm² and the threshold voltage was 5.2V. The lifetimewas 20 hours or longer.

Example 13

The laser devices having the same constructions as those in Example 11were fabricated, except that the second layer composing the n-sidecladding layer 14 consisted of GaN doped with Si to 1×10¹⁷/cm³. Thepresent laser devices had the similar properties to those in Example 11.

Example 14

The laser devices having the same constructions as those in Example 11were fabricated, except that the second layer composing the n-sidecladding layer 14 consisted of undoped GaN. The present laser deviceshad the similar properties to those in Example 11.

Example 15

The laser devices were fabricated in the same manner as in Example 9,except that the 0.4 μm-thick n-side cladding layer 14 consisting of onlyn-type Al_(0.2)Ga_(0.8)N doped with Si to 1×10¹⁹/cm³ was grown. That is,the present laser devices had the basic constructions as described intable 1, except that the p-side cladding layer 19 had a superlatticestructure with a total thickness of 0.4 μm composed of the20-angstrom-thick first layers consisting of p-type Al0.2Ga0.8N dopedwith Mg to 1×10²⁰/cm³ and the 20-angstrom-thick second layers consistingof p-type GaN doped with Mg to 1×10¹⁹/cm³ and further, the p-sidecontact layer 20 had a thickness of 150 angstroms and consisted ofp-type GaN doped with Mg to 2×10²⁰/cm³ like Example 1. The continuousemission at a wavelength of 405 nm was observed at the threshold currentdensity of 3.4 kA/cm². The threshold voltage was 5.1V and the lifetimewas 20 hours or longer.

Example 16

The laser devices were fabricated in the same manner as in Example 15,except that the p-side cladding layer 19 had a superlattice structurewith a total thickness of 0.5 μm obtained by laminating the60-angstrom-thick first layers (Al_(0.2)Ga_(0.8)N) and the40-angstrom-thick second layers (GaN). The threshold voltage tended torise a little. The lifetime was 20 hours or longer.

Example 17

The laser devices were fabricated in the same manner as in Example 9,except that the 0.4 μm-thick p-side cladding layer 19 consisting of onlyp-type Al_(0.2)Ga_(0.8)N doped with Mg to 1×10²⁰/cm³ was grown. That is,the present laser devices had the basic constructions as described intable 1, except that the n-side cladding layer 14 had a superlatticestructure with a total thickness of 0.4 μm composed of the20-angstrom-thick first layers consisting of p-type Al_(0.2)Ga_(0.8)Ndoped with Si to 1×10¹⁹/cm³ and the 20-angstrom-thick second layersconsisting of undoped GaN and further, the p-side contact layer 20 had athickness of 150 angstrom and consisted of p-type GaN doped with Mg to2×10²⁰/cm³ like Example 1. The continuous emission at a wavelength of405 nm was observed at the threshold current density of 3.5 kA/cm². Thethreshold voltage was 5.4V and the lifetime was 20 hours or longer.

Example 18

The laser devices were fabricated in the same manner as in Example 17,except that the n-side cladding layer 14 had a superlattice structurewith a total thickness of 0.49 μm obtained by laminating the70-angstrom-thick first layers consisting of Al_(0.2)Ga_(0.8)N and the40-angstrom-thick second layers consisting of In_(0.01)Ga_(0.99)N dopedwith Si to 1×10¹⁹/cm³. The threshold voltage tended to rise a little,compared with that in Example 16. The lifetime was 10 hours or longer.

Example 19

The laser devices were fabricated in the same manner as in Example 17,except that the n-side cladding layer 14 had a superlattice structurewith a total thickness of 0.5 μm obtained by laminating the60-angstrom-thick first layers consisting of Al_(0.2)Ga_(0.8)N and the40-angstrom-thick second layers consisting of undoped GaN. The thresholdvoltage tended to rise a little, compared with that in Example 17. Thelifetime was 10 hours or longer.

Example 20

With the same procedures as in Example 9, the n-side light waveguidelayer 15 had a superlattice structure with a total thickness of 800angstrom obtained by laminating the 20-angstrom-thick first layersconsisting of undoped GaN and the 20-angstrom-thick second layersconsisting of undoped In_(0.1)Ga_(0.9)N. In addition, the p-side lightwaveguide layer 18 also had a superlattice structure with a totalthickness of 800 angstrom obtained by laminating the 20-angstrom-thickfirst layers consisting of undoped GaN and the 20-angstrom-thick secondlayers consisting of undoped In_(0.1)Ga_(0.9)N. That is, the presentlaser devices had the basic constructions as described in table 1,except that the n-side cladding layer 14, the n-side light waveguidelayer 15, the p-side light waveguide layer 18 and the p-side claddinglayer 19 had a superlattice structure respectively and further, thep-side contact layer 20 had a thickness of 150 angstrom and consisted ofp-type GaN doped with Mg to 2×10²⁰/cm³ like Example 1. The continuousemission at a wavelength of 405 nm was observed at the threshold currentdensity of 2.9 kA/cm². The threshold voltage was 4.4V and the lifetimewas 60 hours or longer.

Example 21

The present example will be described on the basis of LED devices asshown in FIG. 1. With the same procedures as in Example 1, a200-angstrom-thick buffer layer 2 consisting of GaN was grown on asapphire substrate 1, and subsequently, a 5 μm-thick contact layerconsisting of n-type GaN doped with Si to 1×10¹⁹/cm³ was grown. Next, a30-angstrom-thick active layer 4 having a single quantum well structureand consisting of In0.4Ga0.6N was grown.

(p-Side Superlattice Layer)

Next, with the same procedures as in Example 1, a 20-angstrom-thickfirst layer consisting of p-type Al_(0.2)Ga_(0.8)N doped with Mg to1×10²⁰/cm³ was grown, and subsequently, a 20-angstrom-thick second layerconsisting of p-type GaN doped with Mg to 1×10¹⁹/cm³. And then, a p-sidecladding layer 5 having a superlattice structure with a total thicknessof 0.4 μm was grown. The thickness of the p-side cladding layer 4 is notlimited to a particular value and preferably within the range of 100angstrom to 2 μm, more preferably 500 angstrom to 1 μm.

Next, a 0.5 μm-thick p-type GaN layer doped with Mg to 1×10²⁰/cm³ wasgrown on the p-side cladding layer 5. After the growth, the wafer wasremoved out of the reactor and the annealing was performed in the samemanner as in Example 1. Then, the etching was performed from the side ofthe p-side contact layer 6 to expose the surface of the n-side contactlayer 3 on which a n-electrode 9 was to be formed. A 200-angstrom-thickp-electrode consisting of Ni—Au was formed on the almost whole surfaceof the top p-side contact layer 6. A p-pad electrode consisting of Auwas formed on the whole surface electrode 7. A n-side electrode 9consisting of Ti—Al was formed on the exposed surface of the n-contactlayer.

The wafer on which the electrodes were formed in the above-mentioned waywere cut into chips which were 350 by 350 μm square to obtain LEDdevices. The green-emission at a wavelength of 520 nm was observed at Ifof 20 mA and Vf was 3.2V. On the contrary, LED devices having a p-sidecladding layer 5 consisting of only Mg-doped Al_(0.2)Ga_(0.8)N showed Vfof 3.4V. Moreover, The withstand static voltage of the present examplewas two times better.

Example 22

The LEDs were fabricated in the same manner as in Example 21, exceptthat the p-side cladding layer 5 had superlattices with a totalthickness of 0.25 μm obtained by laminating twenty-five50-angstrom-thick first layers and twenty-five 50-angstrom-thick secondlayers consisting GaN doped with Mg to 1×10²⁰/cm³. The LEDs had similarproperties to those in Example 21.

Example 23

The LEDs were fabricated in the same manner as in Example 21, exceptthat the p-side cladding layer 5 had superlattices with a totalthickness of 0.25 μm obtained by laminating 100-angstrom-thick firstlayers and 70-angstrom-thick second. Vf was 3.4V, but The withstandstatic voltage was superior to that of the conventional devices by 20%.

Example 24

With the same procedures as in Example 21, for growing the n-sidecontact layer 3, the 60-angstrom-thick first layer consisting of n-typeAl_(0.2)G₀₈N doped with Si to 2×10¹⁹/cm³ was grown and the40-angstrom-thick second layer consisting of undoped GaN was grown, andfive-hundred first layers and five-hundred second layers were laminatedby turns to obtain superlattices with a total thickness of 5 μm. TheLEDs were fabricated in which other constructions were the same as thosein Example 12. Vf decreased to 3.1V at If of 20 mA. The withstand staticvoltage was 2.5 times better than that of the conventional LEDs.

Example 25

The LEDs were fabricated in the same manner as in Example 23, exceptthat the p-side cladding layer 5 was composed of superlattices with atotal thickness of 0.3 μm obtained by laminating twenty-five60-angstrom-thick first layers (Al_(0.2)Ga_(0.8)N) and twenty-five40-angstrom-thick second layers, by turns. Vf was 3.2V and The withstandstatic voltage was two times better than that of the conventional LEDs.

Example 26

First, a 300 μm-thick GaN layer doped with Si to 5×10¹⁸/cm³ was grown ona sapphire substrate using MOVPE method or HVPE method, and then thesapphire substrate was removed to fabricate a Si-doped GaN substrate 101having a thickness of 300 μm. The GaN substrate 101 was obtained bygrowing a nitride semiconductor layer to a thickness of 100 μm or moreon a substrate which is not a nitride semiconductor and by removing thesubstrate. The GaN substrate may be made of undoped GaN or n-typeimpurity -doped GaN. In the case of doping a n-type impurity, theimpurity was usually doped within the range of 1×10¹⁷/cm³ to 1×10¹⁹/cm³to obtain a GaN substrate having a few defect.

After the fabrication of the GaN substrate 101, the temperature wasadjusted to 1050° C. and a 3 μm-thick third buffer layer 113 consistingof n-type GaN doped with Si to 3×10¹⁸/cm³ was grown. The third bufferlayer corresponds to the n-side contact layer 14 as shown in FIGS. 1 and2. However, an electrode is not on the buffer layer and thus, the thirdbuffer layer 3 is not referred to a contact layer. The first layer maybe grown at a low temperature in the same manner as in Example 1 betweenthe GaN substrate 101 and the third buffer layer 113 and if the firstlayer is grown, the thickness may be preferably 300 angstrom or less.

The 500-angstrom-thick crack-preventing layer 13 consisting ofIn_(0.1)Ga_(0.9)N doped with Si to 5×10¹⁸/cm³ of Si was grown on thethird buffer layer 113 in the same manner as in example 1.

Next, a n-side cladding layer was grown which was composed ofsuperlattices with a total thickness of 0.4 μm obtained by laminating20-angstrom-thick first layers consisting of n-type Al_(0.2)Ga_(0.8)Ndoped with Si to 5×10¹⁸/cm³ and 20-angstrom-thick second layersconsisting of GaN doped with Si to 5×10¹⁸/cm³ by turns 100 times.

Next, a 0.1 μm-thick n-side light waveguide layer 15 consisting ofn-type GaN doped with Si to 5×10¹⁸/cm³ was grown in the same manner asin Example 1.

Next, a 25-angstrom-thick well layer consisting of undopedIn_(0.2)Ga_(0.8)N was grown and a 50-angstrom-thick barrier layerconsisting of undoped GaN was grown. They were grown by turns,respectively two times. And finally, a well layer was grown on the top,with the result that an active layer 16 having amulti-quantum-well-structure (MQW) with a total thickness of 175angstroms was grown.

Next, in the same manner as in Example 1, a p-side cap layer 17consisting of p-type Al_(0.3)Ga_(0.7)N doped with Mg to 1×10²⁰/cm³ wasgrown to a thickness of 300 angstroms and a p-side light waveguide layer18 consisting of p-type GaN doped with Mg to 1×10²⁰/cm³ was grown to athickness of 0.1 μm.

Next, in the same manner as in Example 1, a p-side cladding layer 19 wasformed which was composed of superlattices with a total thickness of 0.4μm obtained by laminating 20-angstrom-thick first layers consisting ofp-type Al_(0.2)Ga_(0.8)N doped with Mg to 1×10²⁰/cm³ of Mg and20-angstrom-thick second layers consisting of p-type GaN doped with Mgto 1×10²⁰/cm³. And finally, a 150-angstrom-thick p-side contact layer 20consisting of p-type GaN doped with Mg to 2×10²⁰/cm³ was grown on thep-side cladding layer 19.

After the reaction was completed, the annealing at 700° C. wasperformed. Then, in the same manner as in Example 1, the top p-sidecontact layer 20 and p-side cladding layer were etched into aridge-geometry with a stripe width of 4 μm with the RIE apparatus.

Next, in the same manner as in Example 1, a p-electrode 21 consisting Niand Au was formed on the almost whole surface of the stripe ridge of thep-side contact layer 20 and a n-electrode 23 consisting of Ti and Al wasformed on the almost back surface of the GaN substrate 101.

Next, as shown in FIG. 6, an insulating layer 25 was formed on thep-side cladding layer 19 except for the p-electrode 21 and a p-padelectrode was formed which connected to the p-electrode 21 electricallyvia the insulating layer 25.

After formation of the electrode, the GaN substrate 101 was cleaved intobars perpendicularly respect to the p-electrode 21 to fabricate facetsof a resonator on the cleaved facet. The cleaved facet of the GaNsubstrate was M plane. A dielectric multi-layer film consisting SiO₂ andTiO₂ was formed on the cleaved facet and finally, the bar was cutparallel to the p-electrode, resulting in laser chips as shown in FIG.6. Next, the chips were set face-up (in the state that the substrate wasfaced to the heat sink) onto the heat sink and the p-pad electrode 22was connected by wire-bonding. The laser was tried at room temperature.The continuous emission at a wavelength of 405 nm was observed at thethreshold current density of 2.5 kA/cm² and the threshold voltage of4.0V at room temperature. The lifetime was 500 hours or longer. Thisresults from usage of GaN substrate to reduce the number of threadingdislocations.

The examples of the present invention will be described, optionally inconnection with the following drawings. FIG. 4 is a perspective viewshowing the shape of the laser devices as shown in FIG. 3.

Example 27

A GaN substrate 100 was prepared by growing a single crystal consistingof GaN to a thickness of 50 μm on a buffer layer consisting of GaN on aC-plane sapphire substrate. The GaN substrate 100 was set within thereactor and the temperature was increased to 1050° C. And using hydrogenas a carrier gas, ammonia and TMG (trimethylgallium) as a source of GaNand silane gas as a source of impurity, a 4 μm-thick n-side buffer layer11 consisting of GaN doped with Si to 1×10¹⁸/cm³ was grown on the GaNsubstrate 100. The buffer layer also acts as a contact layer for formingthe n-electrode when light emitting devices having a structure as shownin FIG. 3 are fabricated. Further, the n-side buffer layer is that isgrown at a high temperature and is distinguished from a buffer layerhaving a thickness of 0.5 μm or less which is grown at a low temperatureof 900° C. or less directly on the substrate made of the material, suchas sapphire, SiC and spinel, which is different from nitridesemiconductors.

(n-Side Cladding Layer 14=Superlattice Layer)

Subsequently, at 1050° C., a 40-angstrom-thick first layer consisting ofn-type Al_(0.2)Ga_(0.9)N doped with Si to 1×10¹⁹/cm³ was grown using TMA(trimethylammonium), TMG, ammonia and silane gas. And then, silane gasand TMA were stopped and a 40-angstrom-thick second layer consisting ofundoped GaN was grown. A superlattice layer was constructed in a mannerof the first layer+the second layer+the first layer+the second layer+ .. . . One-hundred first layers and one-hundred second layers werelaminated by turns. Thus, the n-side cladding layer 14 composed ofsuperlattices having a total thickness of 0.8 μm was grown.

(n-Side Light Waveguide Layer 15)

Subsequently, silane gas was stopped and at 1050° C., a 0.1 μm-thickn-side light waveguide layer 15 consisting of undoped GaN was grown. Then-side light waveguide layer acts as a light waveguide layer of anactive layer and preferably, consists of GaN or InGaN and has athickness of 100 angstroms to 5 μm more preferably of 200 angstroms to 1μm. This n-side light waveguide layer may be composed of undopedsuperlattices. In the case of the superlattice layer, the band gapenergy is larger than that of the active layer and smaller than that ofthe n-cladding layer consisting of Al_(0.2)Ga_(0.8)N.

(Active Layer 16)

Next, an active layer 16 was grown using TMG, TMI and ammonia as asource of InGaN. For the active layer 16, the temperature was maintainedat 800° C. and a 25-angstrom-thick well layer consisting of undopedIn_(0.2)Ga_(0.8)N was grown. Next, at the same temperature, the molarratio of TMI was changed and a 50-angstrom-thick barrier layerconsisting of In_(0.01)Ga_(0.95)N was grown. These operations wererepeated two times. And finally, an well layer was laminated, with theresult that an active layer having a multi-quantum-well (MQW) structurewith a total thickness of 175 angstroms was grown. The active layer maybe undoped like in the present example, or doped with donor impuritiesand/or p-type impurities. Both of the well layer and the barrier layermay be doped with impurities or either of them may be doped.

(p-Side Cap Layer 17)

Next, the temperature was increased to 1050° C. and a 300-angstrom-thickp-side cap layer consisting of p-type Al_(0.3)Ga_(0.7)N doped with Mg to1×10²⁰/cm³ and having a larger band gap energy than the p-side lightwaveguide layer was grown using TMG, TMA, ammonia and Cp2Mg(cyclopentadienyl magnesium). The p-side cap layer has a thickness of0.1 μm or less and the lower limit is not specified, but the thicknessis preferably 10 angstroms or more.

(p-Side Light Waveguide Layer 18)

Subsequently, Cp2Mg and TMA were stopped and at 1050° C., a 0.1 μm-thickp-side light waveguide layer 18 consisting of undoped GaN and having asmaller band gap energy than the p-side cap layer was grown. The layeracts as a light waveguide layer of the active layer and preferablyconsists of GaN or InGaN, like the n-side light waveguide layer 15. Andthe p-side light waveguide layer may be a superlattice layer consistingof a undoped nitride semiconductor or a nitride semiconductor doped withan impurity. In the case of the superlattice layer, the band gap energyis larger than that of the well layer of the active layer and is smallerthan that of the p-side cladding layer consisting of Al_(0.2)Ga_(0.8)N.

(p-Side Cladding Layer)

Subsequently, at 1050° C., a 40-angstrom-thick third layer consisting ofp-type Al_(0.2)Ga_(0.8)N doped with Mg to 1×10²⁰/cm³ was grown. Then,only TMA was stopped and a 40-angstrom-thick fourth layer consisting ofundoped GaN was grown. These operations were repeated, respectively 100times to grow a p-side cladding layer 19 composed of superlattices witha total thickness of 0.8 μm.

(p-Side Contact Layer 20)

Finally, at 1050° C., a 150-angstrom-thick p-side contact layer 20consisting of p-type GaN doped with Mg to 2×10²⁰/cm³ was grown on thep-side cladding layer 19. The p-side contact layer 20 may consist ofp-type In_(x)Al_(y)Ga_(1-x-y)N (0≦X, 0≦Y, X+Y≦1), preferably GaN dopedwith Mg to 2×10²⁰/cm³ to obtain the most preferable ohmic contact to thep-electrode 21. The p-side contact layer 20 was close to the p-sidecladding layer 19 having a superlattice structure including p-typeAl_(y)Ga_(1-y)N and consists of a nitride semiconductor having a smallband gap energy and the thickness of the p-side contact layer 20 was asthin as 500 angstroms or less. Therefore, the carrier density of thep-side contact layer 20 was high and a good ohmic contact between thep-side contact layer and the p-electrode was achieved, with the resultthat the threshold current and voltage of the devices decreased.

The wafer on which the nitride semiconductors were grown in theabove-mentioned manner was annealed at 700° C. within the nitrogenatmosphere in the reactor to make the layers doped with p-typeimpurities less resistive. The annealing method disclosed by U.S. Pat.No. 5,306,662 is employed in this Example.

After annealing, the wafer was removed out of the reactor and as shownin FIG. 3, the top p-side contact layer 20 and p-side cladding layer 19were etched with RIE apparatus into a ridge geometry with a stripe widthof 4 μm. Thus, since the layers above the active layer were made to havea stripe ridge geometry, the emission from the active layer was focusedunder the stripe ridge and the threshold value decreased. Particularly,the layers above the p-side cladding layer 19 composed of superlatticelayers are preferably made to have a ridge geometry.

Next, a mask was formed on the surface of the ridge and the etching wasperformed with a RIE apparatus until the n-side buffer layer wasexposed. The exposed n-side buffer layer 11 also acts as a contact layerfor forming a n-electrode 23. In FIG. 3, the n-side buffer layer 11 isshown as a contact layer. However, the etching can be performed untilthe GaN substrate 100 is exposed and the exposed GaN substrate 100 canbe a contact layer.

Next, a stripe p-electrode 21 consisting of Ni and Au was formed on thetop surface of the ridge of the p-side contact layer 20. The materialsfor the p-electrode 21 to obtain preferable ohmic contact to the p-sidecontact layer may include Ni, Pt, Pd, Ni/Au, Pt/Au, Pd/Au and so on.

On the other hand, a stripe n-electrode 23 consisting of Ti and Au wasformed on the exposed surface of n-side buffer layer 11. The materialsfor the n-electrode 23 to obtain preferable ohmic contact to the GaNsubstrate 100 may include metals such as Al, Ti, W, Cu, Zn, Sn, In andthe like or alloys thereof.

Next, as shown in FIG. 3, an insulating layer 25 was formed on thesurface of the exposed nitride semiconductors between the p-electrodeand n-electrode 23. And a p-pad electrode 22 and n-pad electrode wereformed which were connected to the p-electrode 21 electrically via theinsulating layer 25. The p-pad electrode 22 enlarges the substantialsurface area of the p-electrode 21 to enable the wire-bonding ordie-bonding of the p-electrode. On the other hand, the n-pad electrode24 prevents the n-electrode 23 from coming off.

The wafer on which the n-electrode and p-electrode were formed in theabove-mentioned manner was is transferred to the polishing machine. Thesapphire substrate on whose side the nitride semiconductor was notformed was lapped with an diamond abrasive to a substrate thickness of70 μm. After lapping, the surface of the substrate was further polishedby 1 μm with a finer abrasive, resulting in the mirror facet and thewhole surface was metallized with Au/Sn.

Then, the Au/Sn side was scribed and cleaved into bars perpendicularlyrespect to the strip electrode to fabricate a resonator on the cleavedfacet. A dielectric multi-layer film consisting SiO₂ and TiO₂ was formedon the plane of the resonator and finally, the bar was cut parallel tothe p-electrode, resulting in laser chips. Next, the chips were setface-up (in the state that the substrate was faced to the heat sink)onto the heat sink and each electrode was connected by wire-bonding. Thelaser was tried at room temperature. The continuous emission at awavelength of 405 nm was observed at the threshold current density of 20kA/cm² and the threshold voltage of 4.0V at room temperature. Thelifetime was 100 hours or longer.

Example 28

FIG. 7 is a sectional view showing the structure of the laser devicesaccording to another example of the present invention. In this drawing,the devices which are cut in the perpendicular direction to thepropagating direction of the emission are shown, like in FIG. 3. Example28 will be described with reference to FIG. 7. In FIG. 7, identicalreference numerals have been used to designate identical elements thatare common to FIG. 3 and FIG. 4.

A 150-angstrom-thick single crystal consisting of GaN doped with Si to5×10¹⁸/cm³ was grown via the buffer layer consisting of GaN on theC-plane sapphire substrate, so as to obtain a GaN substrate 100. An-side buffer layer 11 was grown on the GaN substrate in the same manneras in Example 27.

(Crack Preventing Layer 13)

After the growth of the n-side buffer layer 11, the temperature wasadjusted to 800° C. and a 500-angstrom-thick crack preventing layerconsisting of In_(0.1)Ga_(0.9)N doped with Si to 5×10¹⁸/cm³ was grownusing TMG, TMI and ammonia as a source of InGaN and silane gas as asource of Si. The crack preventing layer 13 can be obtained by growing anitride semiconductor containing In, preferably InGaN, and can preventcracks in the nitride semiconductor layers containing Al. The crackpreventing layer may preferably have a thickness ranging 100 angstromsand 0.5 μm. If the crack preventing layer has a thickness less than 100angstroms, the layer is hard to prevent cracks. If the layer has athickness more than 0.5 μm, the crystal itself tends to change intoblack.

After the growth of the crack preventing layer 13, a n-side claddinglayer 14 composed of modulation-doped superlattices 14 and an undopedn-side light waveguide layer 15 were grown in the same manner as inExample 27.

(n-Side Cap Layer 20)

Subsequently, a 300-angstrom-thick n-side cap layer 20 consisting ofn-type Al_(0.3)Ga_(0.7)N doped with Si to 5×10¹⁸/cm³ and having a largerband gap energy than the n side light waveguide layer 15 was grown usingTMG, TMA, ammonia and silane gas.

After this, with the same procedures as those in Example 27, a p-sidecap layer 17, an undoped p-side light waveguide layer 18, a p-sidecladding layer 19 composed modulation-doped superlattices and a p-sidecontact layer 20 were grown.

After the growth of nitride semiconductor layers, annealing wasperformed in the same manner to make the p-type impurities doped layersless resistive. After annealing, as shown in FIG. 7, the top p-sidecontact layer 20 and p-side cladding layer were etched into a ridgegeometry with a stripe width of 4 μm.

After forming a ridge, a stripe p-electrode 21 consisting of Ni/Au wasformed on the top surface of the ridge of the p-side contact layer 20.An insulating layer 25 consisting of SiO₂ was formed on the surface ofthe top nitride semiconductor layer except for the p-electrode 21. Ap-pad electrode 22 was formed which was connected electrically to thep-electrode 21 via the insulating layer 25.

The wafer on which the p-electrode was formed in the above-mentionedmanner was transferred to the polishing machine. The sapphire substratewas removed by polishing to expose the surface of the GaN substrate 100.An n-electrode 23 consisting of Ti/Al was formed on the almost wholesurface of the exposed GaN substrate.

After forming the electrodes, the GaN substrate was cleaved with respectto M-plane (which corresponds to a side plane of a hexagonal column inthe case that the nitride semiconductor is represented according to ahexagonal system). The dielectric multilayers consisting of SiO₂ andTiO₂ were formed on the cleaved facet. The bar was cut parallel to thep-electrode into the laser chips. For these chips, a continuous emissionat room temperature was observed. These devices had similar propertiesto those according to Example 27.

Example 29

With the same procedures as in Example 27, after growing the n-sidebuffer layer 11, a crack-preventing layer 13 was grown in the samemanner as in Example 28. Next, a 0.4-μm-thick n-side cladding layer 14composed of a single layer consisting of Al_(0.3)Ga_(0.7)N doped with Sito 1×10¹⁹/cm³ was grown on the crack preventing layer. The otherconstructions were the same as those in Example 27. For the fabricatedlaser devices, the laser was observed at room temperature and thelifetime was a little shorter than that of the laser devices in Example27.

Example 30

The laser devices were fabricated in the same manner as in Example 27except that the p-side cladding layer 19 was formed by growing a0.4-μm-thick single layer consisting of Al_(0.3)Ga_(0.7)N doped with Mgto 1×10²⁰/cm³. The laser was observed at room temperature and thelifetime was a little shorter than that of the laser devices in Example27.

Example 31

With the same procedures as in Example 27, instead of the n-sidecladding layer 14 having a superlattice structure, a 0.4-μm-thick n-sidecladding layer 14 consisting of Al_(0.2)Ga_(0.8)N doped with Si to1×10¹⁸/cm³ was grown. And instead of the p-side cladding layer having asuperlattice structure, a 0.4-μm-thick p-side cladding layer consistingof Al_(0.2)Ga_(0.8)N doped with Mg to 1×10²⁰/cm³ was grown. The n-sidelight waveguide layer 15 was composed of superlattices with a totalthickness of 0.12 μm obtained by laminating 30-angstrom-thick layersconsisting of GaN doped with Si to 1×10¹⁷/cm³ and 30-angstrom-thicklayers consisting of undoped In_(0.01)Ga_(0.99)N. The p-side lightwaveguide layer 18 was composed of superlattices with a total thicknessof 0.12 μm obtained by laminating 30-angstrom-thick layers consisting ofundoped In_(0.01)Ga_(0.99)N and 30-angstrom-thick layers consisting ofGaN doped with Mg to 1×10¹⁷/cm³ of Mg. The other constructions of thefabricated laser devices were the same as those in Example 27. The laserwas observed at room temperature and the lifetime was a little shorterthan that of the laser devices in Example 27.

Example 32

With the same procedures as in Example 27, the n-side buffer layer 11was composed of superlattices with a total thickness of 1.2 μm obtainedby laminating 30-angstrom-thick undoped GaN layers and 30-angstrom-thicklayers consisting of Al_(0.05)Ga_(0.95)N doped with Si to 1×10¹⁹/cm³.The layers above the n-side cladding layer were grown in the same manneras in Example 27 and the laser devices were fabricated. When then-electrode was formed, the surface exposed by etching was positionedsomewhere of the superlattices with a total thickness 1.2 μm and then-electrode was formed on the exposed superlattice layer. A continuousemission was observed and the threshold value was a little lower thanthat of the laser devices in Example 27. The lifetime was 1000 hours orlonger.

Example 33

FIG. 8 is a sectional view showing the structure of the laser devicesaccording to another example of the present invention. In this drawing,identical reference numerals have been used to designate identicallayers that are common to other drawings. Example 33 will be describedwith reference to FIG. 8.

With the same procedures as in Example 27, a 200-angstrom-thick GaNbuffer layer (not shown) was grown on a (0001) C-plane sapphiresubstrate 30 with 2-inch Φ, at 500° C. and then, a 5 μm-thick undopedGaN layer 31 was grown at 1050° C. The thickness is not limited to 5 μmand may be preferably over the thickness of the buffer layer and 10 μmor less. The material of the substrate may include sapphire, SiC, ZnO,spinel, or other materials which are different from nitridesemiconductors and are known for growing nitride semiconductors such asGaAs.

Next, after growing the undoped GaN layer 31, the wafer was removed outof the reactor. A striped photomask was formed on the surface of the GaNlayer 31 and with a CVD apparatus, 0.1-μm-thick SiO₂ protective film 32was patterned to form 20-μm-wide stripe windows with a periodicity of 5μm. FIG. 8 is a sectional view showing the partial structure of thewafer when cut perpendicularly to the longitudinal direction of thestripes. The examples of the mask pattern may include stripes, dots orchecks and the exposed parts of the undoped GaN layer 31, that is, theparts on which the mask was not formed (windows) may preferably have asmaller area than the mask, so as to grow a GaN substrate 100 with lesscrystal defects. The materials of the mask may include oxides such assilicon oxide (SiO_(x)), silicon nitride (Si_(x)Ni_(y)), titanium oxide(TiO_(x)) and zirconium oxide (ZrO_(x)), nitrides, or multi-layer filmthereof, as well as metals having a melting point higher than 1200° C.These materials can stand at high temperatures ranging between 600° C.and 1100° C. at which nitride semiconductors can be grown and nitridesemiconductors do not grow or are difficult to grow on the surface ofthe materials.

After forming the protective film 32, the wafer was set in the reactoragain and a 10-μm-thick undoped GaN layer to be a GaN substrate 100 wasgrown at 1050° C. The preferable thickness of the GaN layer depends onthe thickness and the size of the protective film 32 and the GaN had athickness enough to grow laterally (perpendicularly to the thicknessdirection) above the mask, so as to cover the surface of the protectivefilm 32. In the case that the GaN substrate 100 was grown in a mannerthat a GaN layer was laterally grown on the surface of the protectivefilm 32 on which nitride semiconductor were difficult to grow,initially, the GaN layer did not grow on the protective film 32 and grewselectively on the undoped GaN layer 31 in the window regions. In thecase that the GaN layer was continued to grow, the GaN layer grewlaterally and covered the protective film 32 and the GaN layers grownfrom the neighboring windows linked with each other, resulting in thatthe GaN layer was grown on the protective film 32. That is, the GaNlayer was laterally grown on the GaN layer 31 via the protective film32. Crystal defects in the GaN layer 31 grown on the sapphire substrate30 and the number of crystal defects in the GaN substrates 100 grown onthe protective film 32 were important. That is, extremely large numberof crystal defects were caused in the nitride semiconductor layers grownon an auxiliary substrate due to the lattice mismatch between theauxiliary substrate and the nitride semiconductors and the crystaldefects threaded through the nitride semiconductors grown sequentiallyupward to the surface. On the other hand, in Example 33, The GaNsubstrate 100 grown laterally on the protective film 32 was not directlyon an auxiliary substrate and was obtained in a manner that the GaNlayer grown from the neighboring windows linked to each other duringgrowing laterally on the protective film 32. Therefore, the number ofcrystal defects decreased extremely, compared with that in thesemiconductor layers grown on an auxiliary substrate. Thus, a mask waspartially formed on the nitride semiconductor layer grown on anauxiliary substrate and a GaN layer was grown laterally on the mask,resulting in a GaN substrate having much less crystal defects than theGaN substrate according to Example 27. Actually, the number of thecrystal defects in the undoped GaN layer was 10¹⁰/cm² or more, while thenumber of crystal defects in the GaN substrate according to Example 33was 10⁶/cm² or less.

After forming a GaN substrate 100 in the above-mentioned manner, a5-μm-thick n-side buffer layer, besides acting as a contact layer 11,consisting of GaN doped with Si to 1×10¹⁸/cm³ was grown on the GaNsubstrate in the same manner as in Example 27. And then, a500-angstrom-thick crack preventing layer 13 consisting ofIn_(0.1)Ga_(0.9)N doped with Si to 5×10¹⁸/cm³ was grown. The crackpreventing layer 13 can be omitted.

(n-Side Cladding Layer 14 Composed of Superlattices Having a HighlyDoped Center Part)

Next, a 20-angstrom-thick undoped GaN layer was grown using TMG andammonia gas at 1050° C. to form a second nitride the same temperature,adding TMA, a 5-angstrom-thick undoped Al_(0.1)Ga_(0.9)N layer was grownand subsequently, adding silane gas, a 20-angstrom-thickAl_(0.1)Ga_(0.9)N layer doped with Si to 1×10¹⁹/cm³ was grown, then,stopping Si, a 5-angstrom-thick undoped Al_(0.1)Ga_(0.9)N layer beinggrown to form a 30-μm-thick first nitride semiconductor layer having alarge band gap energy. Thereafter, in the same manner, the secondnitride semiconductor layers and the first nitride semiconductor layerswere formed by turns repeatedly. In Example 33, one-hundred-twentysecond layers and one-hundred-twenty first layers were laminated byturns to form a n-side cladding layer 14 of superlattices with athickness of 6 μm.

Next, a n-side light waveguide layer 15, an active layer 16, a p-sidecap layer 17, and a p-side light waveguide layer were grown sequentiallyin the same manner as in Example 27.

(p-Side Cladding Layer 19 Composed of Superlattices Having a HighlyDoped Center Part)

Next, a 20-angstrom-thick undoped GaN layer was grown using TMG andammonia gas at 1050° C. to form a fourth nitride semiconductor layerhaving a small band gap energy. Next, at the same temperature, addingTV, a 5-angstrom-thick undoped Al_(0.1)Ga_(0.9)N layer was grown andsubsequently, adding Cp2Mg, a 20-angstrom-thick Al_(0.1)Ga_(0.9)N layerdoped with Mg to 1×10²⁰/cm³ was grown, then, stopping Cp2Mg, a5-angstrom-thick undoped Al_(0.1)Ga_(0.9)N layer being grown to form a30-μm-thick third nitride semiconductor layer having a large band gapenergy. Thereafter, in the same manner, the fourth nitride semiconductorlayers and the third nitride semiconductor layers were formed by turnsrepeatedly. In Example 33, one-hundred-twenty fourth layers andone-hundred-twenty third layers were laminated by turns to form a n-sidecladding layer 19 of superlattices with a thickness of 6 μm.

And finally, a p-side contact layer 20 was grown in the same manner asin Example 27 and then, the wafer was removed out of the reactor. Theannealing was performed and the layers above the p-side cladding layerwere etched into a stripe ridge geometry.

Next, as shown in FIG. 8, etching was performed symmetrically withrespect to the ridge to expose the surface of the n-side buffer layer onwhich a n-electrode was to be formed and a n-electrode was formed. Onthe other hand, a stripe p-electrode was also formed on the top surfaceof the ridge of the p-side contact layer 20. Thereafter, in the samemanner as in Example 27, laser devices were fabricated. The thresholdcurrent density and voltage decreased by 10%, compared to those inExample 27. The continuous emission at a wavelength of 405 nm wasobserved for 2000 hours or more. This was mainly because the enhancementof the crystal quality in the nitride semiconductors due to the GaNsubstrate 100 having less crystal defects. In FIG. 8, in the case of theGaN substrate 100 having a thickness of, for example, 80 μm or more, thelayers between an auxiliary substrate 30 and the protective film 32 canbe omitted.

Example 34

With the same procedures as in Example 33, the n-side cladding layer wascomposed of superlattices with a total thickness of 0.6 μm obtained bylaminating 20-angstrom-thick undoped GaN layers and 20-angstrom-thickAl_(0.1)Ga_(0.9)N layers doped with Si to 1×10¹⁹/cm³ instead ofsuperlattices having a highly doped center part.

On the other hand, the p-side cladding layer 19 was composed ofsuperlattices with a total thickness of 0.6 μm obtained by laminating20-angstrom-thick undoped GaN layers and 20-angstrom-thickAl_(0.1)Ga_(0.9)N layers doped with Mg to 1×10²⁰/cm³ instead ofsuperlattices having a highly doped center part. The other constructionswere the same as those in Example 33. For the fabricated laser devices,the threshold value decreased a little and the lifetime was similarly2000 hours or longer, compared to those of Example 33.

Example 35

With the same procedures as in Example 33, the n-side cladding layer 14was composed of superlattices with a total thickness of 0.6 μm obtainedby laminating 25-angstrom-thick GaN layers doped with Si to 1×10¹⁹/cm³and 25-angstrom-thick undoped Al_(0.1)Ga_(0.9)N layers respectively byturns. On the other hand, the p-side cladding layer 19 was composed ofsuperlattices with a total thickness of 0.6 μm obtained by laminating25-angstrom-thick GaN layers doped with Mg to 1×10²⁰/cm³ and25-angstrom-thick undoped Al_(0.1)Ga_(0.9)N layers respectively byturns. The other constructions were the same as those in Example 33. Thefabricated laser devices had similar properties and lifetime to those ofExample 33.

Example 36

With the same procedures as in Example 33, the n-side cladding layer 14was composed of superlattices with a total thickness of 0.6 μm obtainedby laminating 25-angstrom-thick GaN layers doped with Si to 1×10¹⁹/cm³and 25-angstrom-thick Al_(0.1)Ga_(0.9)N layers doped with Si to1×10¹⁷/cm³ respectively by turns. On the other hand, the p-side claddinglayer 19 was composed of superlattices with a total thickness of 0.6 μmobtained by laminating 25-angstrom-thick GaN layers doped with Mg to1×10²⁰/cm³ and 25-angstrom-thick Al_(0.1)Ga_(0.9)N layers doped with Mgto 1×10¹⁸/cm³ respectively by turns. The other constructions were thesame as those in Example 33. The fabricated laser devices had similarproperties and lifetime to those of Example 33.

Example 37

With the same procedures as in Example 33, the n-side cladding layer wascomposed of a 0.6 μm-thick Al_(0.1)Ga_(0.9)N layer doped with Si to1×10¹⁹/cm³ instead of superlattices. On the other hand, the p-sidecladding layer 19 was composed of superlattices with a total thicknessof 0.6 μm obtained by laminating 25-angstrom-thick GaN layers doped withMg to 1×10²⁰/cm³ and 25-angstrom-thick Al_(0.1)Ga_(0.9)N layers dopedwith Mg to 1×10¹⁸/cm³ respectively by turns. The other constructionswere the same as those in Example 33. For the fabricated laser devices,the threshold values increased a little and the lifetime was similarly1000 hours or longer, compared to those of Example 33.

Example 38

With the same procedures as in Example 33, the n-side cladding layer andp-side cladding layer were modulation-doped superlattices (in which thecenter part was not highly doped and the impurity density was almosteven within the layer). And the n-side buffer layer 11 was composed ofsuperlattices with a total thickness of 2 μm obtained by laminating50-angstrom-thick Al_(0.05)Ga_(0.95)N layers doped with Si to 1×10¹⁹/cm³and 50-angstrom-thick undoped GaN layers respectively by turns. Theother constructions were the same as those in Example 33. For thefabricated laser devices, the threshold values decreased a little andthe lifetime was 3000 hours or longer, compared to those of Example 33.

Example 39

With the same procedures as in Example 33, the n-side cladding layer 14was composed of superlattices with a total thickness of 0.6 μm obtainedby laminating 20-angstrom-thick undoped GaN layers and 20-angstrom-thickAl_(0.1)Ga_(0.9)N layers doped with Si to 1×10¹⁹/cm³ respectively byturns. The n-side light waveguide layer 15 was composed of superlatticeswith a total thickness of 0.1 μm obtained by laminating25-angstrom-thick GaN layers doped with Si to 1×10¹⁹/cm³ and25-angstrom-thick undoped Al_(0.05)Ga_(0.95)N layers respectively byturns.

On the other hand, the p-side light waveguide layer was composed ofsuperlattices with a total thickness of 0.1 μm obtained by laminating25-angstrom-thick GaN layers doped with Mg to 1×10¹⁹/cm³ and25-angstrom-thick undoped Al_(0.05)Ga_(0.95)N layers respectively byturns. Next, the p-side cladding layer was composed of superlatticeswith a total thickness of 0.6 μm obtained by laminating20-angstrom-thick undoped GaN and 20-angstrom-thick Al_(0.1)Ga_(0.9)Nlayers doped with Mg to 1×10²⁰/cm³ respectively by turns. The otherconstructions were the same as those in Example 33. For the fabricatedlaser devices, the threshold values decreased a little and the lifetimewas 3000 hours or longer, compared to those of Example 33.

Example 40

Example 40 provides laser devices fabricated using a GaN substrate likeExample 33.

That is, the laser devices according to Example 40 were fabricated byforming the following semiconductor layers on the GaN substrate 100which was formed in the same manner as in Example.

First, a 2-μm-thick n-side contac-t layer (n-side second nitridesemiconductor layer) consisting of n-type GaN doped Si to 1×10¹⁸/cm³ ormore was grown on the GaN substrate 100. This layer may be composed ofsuperlattices consisting of undoped GaN and Si doped Al_(x)Ga_(1-x)N(0<X≦0.4).

Next, after growing the n-side contact layer, at 800° C., using TMG,TMI, ammonia and silane gas in the nitrogen atmosphere, a500-angstrom-thick crack preventing layer consisting ofIn_(0.1)Ga_(0.9)N doped with Si to 5×10¹⁸/cm³ was grown. Thecrack-preventing layer consisted of n-type nitride semiconductorcontaining In, preferably InGaN, so as to prevent the crack fromthreading the nitride semiconductor layers containing Al to be grownthereafter. The thickness of the crack preventing layer may preferablyrange between 1000 angstroms and 0.5 μm. If the thickness is thinnerthan 100 angstroms, the crack preventing layer is difficult to preventcracks. If the thickness is thicker than 0.5 μm, the crystal itselftends to turn into black.

Subsequently, at 1050° C., using TMA, TMG, ammonia and silane gas, a40-angstrom-thick n-type Al_(0.2)Ga_(0.8)N layer doped with Si to1×10¹⁹/cm³ and a 40-angstrom-thick undoped GaN layer were grown.One-hundred of these layers were laminated respectively by turns to growa n-side cladding layer of superlattices with a total thickness of 0.8μm.

Subsequently, a 0.1 μm thick n-side light waveguide layer consisting ofundoped Al_(0.05)Ga_(0.95)N was grown. This layer acts as a lightwaveguide layer to guide the waves of light emitted from the activelayer and may be undoped or doped with n-type impurity. This layer maybe composed of superlattices consisting of GaN and AlGaN.

Next, a 400-angstrom-thick active layer consisting of undopedIn_(0.01)Ga_(0.99)N was grown.

Next, a 300-angstrom-thick p-side cap layer consisting of p-typeAl_(0.2)Ga₀, N doped with Mg to 1×10¹⁹/cm³ and having a larger band gapenergy than the p-side light waveguide layer to be formed thereafter wasgrown.

Next, a 0.1 μm-thick n-side light waveguide layer consisting ofAl_(0.01)Ga_(0.99)N and having a smaller band gap energy than the p-sidecap layer was grown. This layer acted as a waveguide for the activelayer. The layer may be composed of superlattices consisting of nitridesemiconductors. In the case that the p-side light waveguide layer iscomposed of superlattices, the band gap energy of the barrier layerhaving a larger band gap energy should larger than that of the activelayer and smaller than that of the p-side cladding layer.

Subsequently, a p-side cladding layer composed of superlattices with atotal thickness of 0.8 μm obtained by laminating 40-angstrom-thickp-type Al_(0.2)Ga_(0.8)N layers doped with Mg to 1×10¹⁹/cm³ and40-angstrom-thick undoped GaN layers respectively by turns was grown.

Finally, a 150-angstrom-thick p-side contact layer consisting of p-typeGaN doped with Mg to 1×10²⁰/cm³ was grown on the p-side cladding layer.Particularly, in the case of laser devices, the p-Side contact layerconsisting of nitride semiconductors having a small band gap energy isin contact with the p-side cladding layer of superlattices containingAlGaN and has a thickness of as thin as 500 angstroms or less.Therefore, the carrier density in the p-side contact layer is high and agood ohmic contact to the p-electrode is obtained, with the result thatthe threshold current and voltage tends to decrease.

The wafer on which nitride semiconductors were grown in theabove-mentioned manner was annealed at a given temperature to make thep-type impurity doped layers less resistive and removed out of thereactor. The top p-side contact layer and the p-side cladding layer wereetched into a ridge geometry with a stripe width of 4 μm. Thus, thelayers above the active layer had a stripe ridge geometry and theemission from the active layer focused under the stripe ridge, with theresult that the threshold values decreased. Particularly, the layersabove the p-side cladding layer of superlattices may preferably have aridge geometry. The annealing method disclosed by U.S. Pat. No.5,306,662 is employed in this Example.

Next, a mask was formed on the surface of the ridge and the etching wasperformed with a RIE apparatus to expose the surface of the n-sidecontact layer, a stripe n-electrode consisting of Ti and Al beingformed. On the other hand, a stripe p-electrode consisting of Ni and Auwas formed on the top surface of the ridge of the p-side contact layer.Examples of electrode materials to obtain a good ohmic contact to thep-type GaN layer may include Ni, Pt, Pd, Ni/Au, Pt/Au, Pd/Au and so on.Examples of electrode materials to obtain a good ohmic contact to then-type GaN may include metals such as Al, Ti, W, Cu, Zn, Sn or In andalloys thereof.

Next, insulating layers consisting of SiO₂ were formed on the exposedsurfaces of the nitride semiconductor layers between the p-electrode andthe n-electrode and a p-pad electrode was formed which was connectedwith the p-electrode electrically via the insulating layers. Thep-electrode enlarged substantially the surface area of the p-electrodeto enable wire-bonding and die-bonding of the p-electrode side.

The wafer on which the n-electrode and p-electrode were formed in theabove-mentioned manner is transferred to the polishing machine. Thesapphire substrate on whose side the nitride semiconductors were notformed was lapped with a diamond abrasive to a substrate thickness of 70μm. After lapping, the surface of the substrate was further polished by1 μm with a finer abrasive, resulting in the mirror facet and the wholesurface was metallized with Au/Sn.

Then, the Au/Sn side was scribed and cleaved into bars perpendicularlywith respect to the stripe electrode to fabricate a facet of a resonatoron the cleaved facet. A dielectric multi-layer film consisting SiO₂ andTiO₂ was formed on the facet of the resonator and finally, the bar wascut parallel to the p-electrode, resulting in laser chips. Next, thechips were set face-up (in the state that the substrate was faced to theheat sink) onto the heat sink and each electrode was connected bywire-bonding. The laser oscillation was tried at room temperature. Thecontinuous emission at a wavelength of 368 nm was observed at thethreshold current density of 2.0 kA/cm² and the threshold voltage of4.0V at room temperature. The lifetime was 1000 hours or longer.

Example 41

The laser device in Example 41 and the following examples was fabricatedon the basis of the embodiment 4. The Example 41 will be described inconnection with FIG. 9.

(Underlying Layer 302)

The auxiliary substrate 301 made of sapphire with 20-inch Φ and aC-plane was set in the MOVPE reactor and the temperature was set to be500° C. A 200-angstrom-thick buffer layer (not shown) made of GaN wasgrown using trimethylgallium (TMG) and ammonia (NH3). After the growthof the buffer layer, the temperature was increased to 1050° C. and a4-μm-thick underlying layer 302 made of the same GaN was grown.

(Protective Film 303)

After the growth of the underlying layer 302, the wafer was removed outof the reactor and a stripe-shaped photomask was formed on the surfaceof the underlying layer. A 1-m-thick protective film 303 made of SiO₂was patterned to form 10-μm-wide stripe windows with a periodicity of 2μm on the underlying layer 302 with a CVD apparatus.

(Nitride Semiconductor Substrate 304)

After the growth of the protective film 303, the wafer was set again inthe MOVPE reactor and the temperature was set to be 1050° C. A20-μm-thick nitride semiconductor substrate made of undoped GaN wasgrown using TMG and ammonia. On the surface of the resulting nitridesemiconductor substrate, the crystal defects developed parallel to thestripe-shaped protective film in the stripe center part of theprotective film and in the stripe center part of the window. However,when the ridge of the laser device was formed later, the ridge stripewas formed not to be on the crystal defects, so as to prevent thedeveloping of the crystal defects into the active layer and enhance thereliability of the device.

(n-Side Buffer Layer 311 which Also Functions as a n-Side Contact Layer)

Next, a 5-μm-thick n-side buffer layer made of GaN doped with Si to3×10¹⁸/cm³ was grown on the second nitride semiconductor layer 4 usingammonia and TMG, and silane gas as an impurity gas.

(Crack Preventing Layer 312)

Next, a 0.15-m-thick crack preventing layer 312 made ofIn_(0.16)Ga_(0.84)N was grown using TMG, TMI (trimethylindium) andammonia at 800° C.

(n-Side Cladding Layer 313=a Super Lattice Layer)

Subsequently, a 25-angstrom-thick first layer made of n-typeAl_(0.16)Ga_(0.84)N doped with Si to 1×10¹⁸/cm³ was grown using TMA,TMG, ammonia and silane gas at 1050° C. Successively, the silane gas andTMA was stopped and a 25-angstrom-thick second layer made of undoped GaNwas grown. The first layer and the second layer may be laminated in theorder of 1st, 2nd, 1st and so on, to constitute a super lattice layer,resulting in the n-side cladding layer 313 having a super latticestructure and a total thickness of 1.2 μm. The n-side cladding layermade in a super lattice structure had an Al average composition of 8.0%and the product thereof multiplied by the thickness was 9.6.

(n-Side Light Waveguide Layer 314)

Subsequently, the silane gas was stopped and a 0.1-μm-thick n-side lightwaveguide layer made of undoped GaN was grown at 1050° C.

(Active Layer 315)

Next, the active layer 314 was grown using TMG, TMI and ammonia. For theactive layer, the temperature was maintained at 800° C. and a40-angstrom-thick quantum well layer made of undoped In_(0.2)Ga_(0.8)Nwas grown. Successively, only the ratio of TMI was changed and at thesame temperature, a 100-angstrom-thick barrier layer made of undopedIn_(0.01)Ga_(0.99)N was grown. The quantum well layer and the barrierlayer were laminated alternately so that a barrier layer was the lastone, resulting in the active layer having a multi-quantum-well structureand a total thickness of 440 angstroms.

(p-Side Cap Layer 316)

Next, the temperature was increased to 1050° C. and a 300-angstrom-thickp-side cap layer 316 made of p-type Al_(0.3)Ga_(0.7)N doped Mg to1×10²⁰/cm³ and has a larger band gap energy than the p-side lightwaveguide layer 317 was grown using TMG, TMA, ammonia an Cp2Mg(cyclopentadienyl magnesium).

(p-Side Light Waveguide Layer 317)

Subsequently, Cp2Mg and TMA were stopped and a 0.1-μm-thick p-side lightwaveguide layer made of undoped GaN and having a smaller band gap energythan the p-side cap layer 316 was grown at 1050° C.

(p-Side Cladding Layer 318)

Subsequently, a 25-angstrom-thick third layer made of p-typeAl_(0.16)ga_(0.84)N doped with Mg to 1×10²⁰/cm³ was grown at 1050° C.Successively, only TMG was stopped and a 25-angstrom-thick fourth layermade of undoped GaN was grown, so as to obtain the p-side cladding layerhaving a super lattice layer with a total thickness of 0.6 μm. Thep-side cladding layer also had an Al average composition of 8% and theproduct thereof multiplied by the thickness was 4.8.

(p-Side Cladding Layer 319)

Subsequently, a 25-angstrom-thick third layer made of p-typeAl_(0.16)Ga_(0.84)N doped with Mg to 1×10²⁰/cm³ was grown at 1050° C.and successively, only TMA was stopped and a 25-angstrom-thick fourthlayer made of undoped GaN was grown, so as to grow the p-side claddinglayer 318 having a super lattice structure with a total thickness of 0.6μm. The p-side cladding layer had an average Al composition of 8% andthe product thereof multiplied by the thickness was 4.8.

(p-Side Contact Layer 319)

Finally, a 150-angstrom-thick p-side contact layer 318 made of p-typeGaN doped with Mg to 2×10²⁰/cm³ was grown on the p-side cladding layer318 at 1050° C.

The wafer on which the nitride semiconductor was grown in theabove-mentioned way was annealed in the nitrogen atmosphere within thereactor to decreased the resistance of the layer doped with a p-typeimpurity.

After annealing, the wafer was removed out of the reactor and the p-sidecontact layer 318 which was the top layer and the p-side cladding layer317 were etched into a ridge having a stripe width of 4 μm, as shown inFIG. 9, with a RIE apparatus. When the ridge stripe was formed, saidridge was formed at the position where the crystal defects did notdevelop on the surface of the nitride semiconductor substrate.

Next, a mask was formed on the ridge surface and the etching wasperformed to expose the surface of the n-side buffer layer 311.

Next, the p electrode 320 made of Ni and Au was formed in the form ofstripe on the outermost surface of the ridge of the p-side contact layer319, while the n electrode 322 made of Ti and Al was formed in the formof stripe on the surface of the n-side buffer layer 311 which had beenjust exposed. Thereafter, as shown in FIG. 9, an insulating film 323made of SiO₂ was formed on the surface of the nitride semiconductorlayer which were exposed between the p electrode 320 and the n electrode322. The p pad electrode 321 was formed which was electrically connectedto the p electrode 320 via said insulating layer 323.

The sapphire substrate of the wafer on which the n electrode and pelectrode were formed in the above-mentioned manner was polished to athickness of 70μ, and then the wafer was cleaved into barsperpendicularly to the stripe-geometry electrode from the substrate sideto fabricate a facet of a resonator on the cleaving facet. A multi-layerdielectric film consisting SiO₂ and TiO₂ was formed on the facet of theresonator and finally, the bar was cut parallel to the p-electrode,resulting in laser chips.

The laser device was set onto the heat sink and each electrode wasconnected by wire-bonding. The laser oscillation was tried at roomtemperature. The continuous emission was observed at room temperature.The FFP of the single laser was uniform and the shape thereof was anoval, which was a good shape. Concerning the characteristics of thelaser device, the threshold was decreased by not less than 10% and thelifetime was increased by not less than 50%, compared with the laserdevice which we had published in Jpn. J. Appl. phys. Vol. 36 (1997).

Example 42

With the same procedures as in Example 41 except that the n-sidecladding layer 313 had a super lattice layer structure with a totalthickness of 1.0 μm which was obtained by laminating a 25-angstrom-thickSi-doped n-type Al_(0.20)Ga_(0.80)N and a 25-angstrom-thick undoped GaN,the laser device was fabricated. The n-side cladding layer had anaverage Al composition of 10.0% and the product thereof multiplied bythe thickness was 10.0. The laser device had almost the samecharacteristics as that in Example 41.

Example 43

With the same procedures as in Example 41 except that the n-sidecladding layer 313 had a super lattice layer structure with a totalthickness of 0.7 μm which was obtained by laminating a 25-angstrom-thickSi-doped n-type Al_(0.20)Ga_(0.80)N and a 25-angstrom-thick undoped GaN,the laser device was fabricated. The n-side cladding layer had anaverage Al composition of 1.0% and the product thereof multiplied by thethickness was 7.0. The laser device had almost the same characteristicsas that in Example 41.

Example 44

With the same procedures as in Example 41 except that the n-sidecladding layer 313 had a super lattice layer structure with a totalthickness of 0.8 μm which was obtained by laminating a 25-angstrom-thickSi-doped n-type Al_(0.12)Ga_(0.88)N and a 25-angstrom-thick undoped GaN,the laser device was fabricated. The n-side cladding layer had anaverage Al composition of 6.0% and the product thereof multiplied by thethickness was 4.8. The threshold of said laser device was decreased bynot less than 5% and the lifetime was increased by not less than 20%,compared with the laser device which we had published in Jpn. J. Appl.phys. Vol. 36 (1997).

Example 45

With the same procedures as in Example 41 except that the n-sidecladding layer 313 had a super lattice layer structure with a totalthickness of 1.4 μm which was obtained by laminating a 25-angstrom-thickSi-doped n-type Al_(0.07)Ga_(0.93)N and a 25-angstrom-thick undoped GaN,the laser device was fabricated. The n-side cladding layer had anaverage Al composition of 3.5% and the product thereof multiplied by thethickness was 4.9. The laser device had almost the same characteristicsas that in Example 41.

INDUSTRIAL APPLICABILITY

As mentioned above, the nitride semiconductor devices according to thepresent invention are composed of superlattice layers in the domain ofthe p-type nitride semiconductor or n-type nitride semiconductor exceptfor the active layer, to improve the electric power efficiencyextremely.

That is, in the conventional nitride semiconductor devices, the activelayer having a multi-quantum-well-structure has been proposed. However,the layer such as the cladding layer, which was on the either side ofthe active layer, has been usually composed of a single nitridesemiconductor layer. In the nitride semiconductor devices according tothe present invention, the cladding layer or the contact layer injectingelectric current is composed of superlattice layer which shows quantumeffect to reduce the resistivity of the cladding layer side. Therefore,the threshold electric current and the threshold voltage of LD devicescan be low and the lifetime thereof can be long. Moreover, theconventional LED has been weak to the static electricity, but thedevices according to the present invention can have a good electricalstatic discharge survivability. Thus, since Vf and the threshold voltagecan be low, the calorific value can be small and the reliability of thedevices can be enhanced. The nitride semiconductor devices according tothe present invention can be applied to emitting devices such as LED andLD, as well as solar cells using nitride semiconductors, light sensors,transistors and so on, to realize devices with very high efficiency andthe industrial applicability is very wide.

1. A nitride semiconductor device comprising an n-side semiconductorregion comprising nitride semiconductor layers, a p-side semiconductorregion comprising nitride semiconductor layers and an active layer of anitride semiconductor between said n-side semiconductor region and saidp-side semiconductor region, wherein said p-side semiconductor regionhas a p-side contact layer having a thickness of not more than 500angstroms, wherein said n-side semiconductor region has an n-side superlattice layer comprising one or more first nitride semiconductor layersand one or more second semiconductor layers each having differentcomposition and different impurity concentration from that of said firstsemiconductor layers and each of said first semiconductor layers andeach of said second semiconductor layers being laminated alternately, atleast one of said first nitride semiconductor layers and said secondnitride semiconductor layers having a thickness of not more than 100angstroms.
 2. The nitride semiconductor device according to claim 1,wherein said p-side semiconductor region has a p-side cladding layerhaving a band gap energy larger than that of said p-side contact layerand said p-side contact layer is formed on said p-side cladding layer.3. The nitride semiconductor device according to claim 1, wherein saidp-side contact layer is formed of one material selected from the groupconsisting of InGaN and GaN.
 4. The nitride semiconductor deviceaccording to claim 1, wherein said p-side contact layer has a thicknessof not more than 200 angstroms.
 5. The nitride semiconductor deviceaccording to claim 1, wherein a transparent electrode is formed on saidp-side contact layer.
 6. The nitride semiconductor device according toclaim 1, wherein the combination of first and second nitridesemiconductor layers is selected from the group consisting of acombination of ternary mixed crystals and a combination of ternary mixedcrystal and binary mixed crystal.
 7. The nitride semiconductor deviceaccording to claim 1, wherein the combination of first and secondnitride semiconductor layers is selected from the group consisting of acombination of In_(X)Ga_(1-X)N (0≦X≦1) and Al_(Y)Ga_(1-Y)N (0≦Y≦1) and acombination of Al_(X)Ga_(1-X)N (0≦X≦1) and Al_(Y)Ga_(1-Y)N (0<Y<1, X<Y).8. The nitride semiconductor device according to claim 1, wherein saidn-side super lattice layer is an n-side cladding layer.
 9. The nitridesemiconductor device according to claim 1, wherein said active layer hasan InGaN layer.
 10. The nitride semiconductor device according to claim9, wherein said InGaN layer is a quantum well layer.
 11. A nitridesemiconductor device comprising an n-side semiconductor regioncomprising nitride semiconductor layers, a p-side semiconductor regioncomprising nitride semiconductor layers and an active layer of a nitridesemiconductor between said n-side semiconductor region and said p-sidesemiconductor region, wherein said p-side semiconductor region has ap-side contact layer having a thickness of not more than 500 angstroms,wherein said p-side semiconductor region has an n-side super latticelayer comprising one or more first nitride semiconductor layers and oneor more second semiconductor layers each having a different compositionand a different impurity concentration from those of said firstsemiconductor layers and each of said first semiconductor layers andeach of said second semiconductor layers being laminated alternately, atleast one of said first nitride semiconductor layers and said secondnitride semiconductor layers having a thickness of not more than 100angstroms.
 12. The nitride semiconductor device according to claim 11,wherein said p-side semiconductor region has a p-side cladding layerhaving a band gap energy larger than that of said p-side contact layerand said p-side contact layer is formed on said p-side cladding layer.13. The nitride semiconductor device according to claim 11, wherein saidp-side contact layer is formed of a material selected from the groupconsisting of InGaN and GaN.
 14. The nitride semiconductor deviceaccording to claim 11, wherein said p-side contact layer has a thicknessof not more than 200 angstroms.
 15. The nitride semiconductor deviceaccording to claim 11, wherein a transparent electrode is formed on saidp-side contact layer.
 16. The nitride semiconductor device according toclaim 11, wherein the combination of first and second nitridesemiconductor layers is selected from the group consisting of acombination of ternary mixed crystals and a combination of ternary mixedcrystals and binary mixed crystal.
 17. The nitride semiconductor deviceaccording to claim 11, wherein the combination of first and secondnitride semiconductor layers is selected from the group consisting of acombination of In_(X)Ga_(1-X)N (0≦X≦1) and Al_(Y)Ga_(1-Y)N (0≦Y≦1) and acombination of Al_(X)Ga_(1-X)N (0≦X≦1) and Al_(Y)Ga_(1-Y)N (0<Y<1, X<Y).18. The nitride semiconductor device according to claim 11, wherein saidn-side super lattice layer is an n-side cladding layer.
 19. The nitridesemiconductor device according to claim 11, wherein said active layerhas an InGaN layer.
 20. The nitride semiconductor device according toclaim 19, wherein said InGaN layer is a quantum well layer.